Idle problem

[Parkofadown]

I recently changed my timing belt, power steering pump and hoses, ignition coil pack wiring harnesses and spit fire ignition coil packs and plugs, when I start my car it goes down to 500 rpms and when I put it in reverse it goes down to 200 and variate to 300 back and forth until it goes to 0 Rpms and dies. Can anyone help ?

[DCaff300ZX]

Welcome to Nico!
Unfortunately having done so many things to the car, it makes for more to check over before looking for other issues but here's some places to start. I'll assume you have double-checked your work, but if not be sure to do so to eliminate that area from your problem.
Finding no issues with your repairs, I'd then check over ALL of your engine connections first for any corrosion inside the connector, and secondly that it is seated correctly with a good boot and wiring. Corrosion is a main enemy of many of our engine connectors especially the injectors and IACV, check these two VERY thoroughly as well as both Coolant Temp sensors. The IACV itself is a notorious problem area for idle issues as it gets sticky inside from buildup over it's lifetime, and also the connectors are notorious for corrosion and the harness is easily abused during repairs...I myself bought a "good" used one to no avail and was forced to fork over the $311 for a new one plus the upgraded harness.
Another thing to check is the PTU given you list yours as an '89/'90 model, which had the old style, recalled PTU. A bad PTU can be an issue for idle as well as the connectors...be sure to check both.
Next up would be vacuum hoses especially considering your repairs, be sure they are all home and seated and in good soft condition...personally one of my first jobs with both of my Z32's was to replace ALL vacuum hoses I could find.
Another thing to do would be a new fuel filter if not done already, and a check of the fuel pressure and fuel pump operation...faulty FPS and regulator can be an issue and are common maintenance needs.
Hope this helps, be sure to post up your results!

[Parkofadown]

I cleaned the throttle position sensor connections and which ones I could see and it helped a little , it only does my problem when I press on my breaks and come to a stop now

[Parkofadown]

Getting my fuel pump changed today.

[NolimitZ32]

If its only doing it when you hit the brakes its either a vacuum leak inside your booster but if your brakes don't feel mushy that's likely not it. Or its something in the idle control which is beyond me because I stay the hell away from autos.

[RubyRed300ZX]

Big time vacuum leak.

[Parkofadown]

I am told it was knock sensor and fuel injectors

[DCaff300ZX]

Those are definitely problem children in this arena, but also be on the hunt for a vacuum leak as mentioned and ANY corroded connectors along the way.