I've just acquired a 2007 Nissan Quest, everything seems to be fine, except when running for about 10 to 15 minutes , transmission jerks when shifting to first or reverse ( from Park position ) ? There's no engine light and also no pending codes ( OBD reader ). Any ideas ?

How about from Neutral ??What's the fluid look like ? Is idle high (got a tach? ) , idle at 1000rpm or more does that ,right ?
A test drive IS 10 or 15 min - did you learn this before acquiring ? Before like paying $$$ ?
Fill in some blanks here , but a full " pan drop , filter & fluid change " - BEST with FACTORY fluid will give you a baseline performance to asess . Seeing any crap in pan , and a visual of the valvebody etc. also may be revealing too.Broken parts can end up in bottom of pan & identified - happened to my J30 trans

Check the motor mounts too, if it only happens between D and R.

Line pressure I forgot to mention too can be checked by tapping into a specific threaded location ( you unscrew a plug) , line pressure solenoid is inside the pan though. Other items like trans temp sensor & TPS are in typical symptom chart , but high idle IS #1 symptom chart tip. And like VStar says ya gotta distinguish if it's a loose motor mount , mybe gets "soft" when hot ??? OR is it trans being HARSH ???

There was no such symptom when i test drive it before buying it . I guess that' I didn't drive it that long. Been changing the fluids 2x already using Valvoline special transmission oil. Still some burnt ( dark ) fluid coming out on contaminants. I guess checking the engine support would be a nice idea too . RPM is 1.2 on starting and settle down to .8 when warmed up ( 2 minutes ). It doesn't act that way for the first 10 to 15 minutes though. Thanks for your ideas Mcgiver and Vstar. I get back after checking the supports.

Ya , the motor will visibly lift up very noticeably on one side in going into R , OR lift the other side when going into D , OR EVEN DO BOTH Is what it can look like with loose/worn/softened motor-mounts!!

p.s. You just unscrewing drain plug ONLY to drain-then-fill ?? OR doing like I describe a " pan drop & inspection " ???

The FSM indicates the '07 has electronic engine mounts. The mounts will go "hard" above 950 RPM and "soft" at idle, so if one or both of them are worn out it will cause pretty much the symptoms you're seeing.

Macgiver, yes there were some small ( 1/16" and smaller ) pcs of broken metals in the catch oil fan, and the jerking ONLY HAPPENS after running for about 10 to 15 minutes or when HOT enough...FROM PARK - TO 1st or REVERSE. I'm trying to pinpoint what solenoid is involved with this positions ( 1st and Reverse gears ) ? There are 3 main ones Black ( on top ), Blue (middle) and Green (bottom ).

https://www.nicoclub.com/service-manual ... 7%2Fat.pdf

See AT-20 for the function chart. They don't show a layout for the Valve Body, but you should be able to identify which solenoid is which from the wire colors in the WD.

I'm no transmission expert , and I was hoping you had Idle line-pressure readings COLD =test press. P,R, N,Dand L then WARM or HOT = test press. P ,R ,N ,and L positions then you read line pressure also usually doing "stall " test of the tork converter .
But I buy ATSG ( Automatic Transmission Service Group ) " TECHTRAN " servc. manual ( $38 ) and they have detailed symptom charts , oil line schematic , block diagram , solenoid ID and locations , and suspected fault & and very well help to direct repair . My trans was RE4R01A (RWD) and ex. of the manual part # TMRE4R01A......Go find your model manual ??
Hope that helps , good you got to see the pan contents - be sure to take many smartphone pics !!
When your finished you may " Party like Charlie Sheen "

Be sure you ARE sure you've ruled out VStar's mention of Electronic Motor mounts - that's new to me BTW.
LIke , your actually seeing & feeling a MARKEDLY more severe shift engagement and NOT loose engine rocking when warm - right ? okay

p.s. ALWAYS re-tork your valve body bolts (usually 10mm) . Do this by a quick back & forth breaking loose FIRST of ALL them bolts - THEN do proper tork TWICE . You break loose because V-body bolts can be " frozen " and NOT have proper " stretch " , or holding pressure- thus keeping V-body and separator plates /& gaskets at proper seal . Pressure in the passages will be proper , will hold and be directed fully and proper to cause SOLID shifting . You DO feel a difference when you re-tork , I always find them loosened over time !!!

macgiver, Vstar, sorry, had to reset my computer password since electronic errors has been done. Anyway, yes I will see and check the V body bolts and the engine mounts to be sure. Had an extra cooler installed ( to try address the issue of jerks when hot ). Thanks a lot guys . I'll get back for some results.

Mcgiver, Vstar,,,, after checking lines and conditions of the 3 main solenoid, putting an external cooler ... Still jerks on 1st and Reverse gears after 15 minutes run. Opening up the transmission fan again to check the Valve body torque pressure. What could be an ideal TORQUE pressure required on the Valve Body ?

I go by the book , those ATSG manuals , they got them on Amazon , you can have one " Primed " to you in a day ???