mechanical problem

[89ka24e]

ok im needing help bad!!!! please someone help i have done a tune up on my 1989 nissan 240 sx new plugs, wires, distributor cap, fuel filter air filter i had went to start it in part its idleing around 7 to 9 hundred rpm's when u put it in drive it drops to 5 hundred rpm's i let it run to reset the fuel settings and had the battery off for 15 mins and nothing i got in it to drive took of alright drove good for 5 mins if that came upon a hill and it dropped rpms and speed and about died car runs perfect and shifts fine just cant find the problem i have put about 300 into it the past 2 days i think im wasting my money on nothing if someone can please help me let me know what i need to buy and put on if not im sadly gonna have to sell this car for 800 bucks obo the car has 140k original miles... thank you for your timebrad wilson