throttle goes up and down @1500 by itself

[dan catalyst]

first off hello all.i just got my first import, a dsm.after getting beat in my camaros so many times by them,i finally broke down and bought one.its a 90 gsx all stock and i love it.
im having a problem with tip in stumble.if i hold the car at 1500 the throttle moves up and down all by itself, by aprox 400 rpm.ive tested the idle switch,tps and iac so im resonably sure non of those are at fault.it idles ok,slight searching and slightly high(1200) at start up then comes down after a few sec.other than that it runs good.it makes the car jerk pretty bad while driving at normal speeds then goes away at higher %tps,and seems to be whats called a 'flat spot' in the throttle.it starts running ok when i apply more than 1/3 throttle and boost starts to build.thanks in advance and sorry to make my first post about a problem!
dan

 

[boosted86]

check your vaccum lines, also check your isc, it on your throttle body.

 

[divinaand_1]

Check your timing if its TDC. slightly off will cause it to do this, had this problem before.

Get a timing light to check if its on time.

 

[anteversion]

pretty sure its ISC. this thing is known to work not so good on our cars and cause very bad idle variation when the engine is cold starting. i've had this problem with mine and get rid of it by removing all this unnecessary stuff ( in my conditions of course). Made an isc fiav block off plate and no more shit with idle variation. By the way its very bad for the engine to start up and reving toomuch like 1500 rpm+ cause oil pressure its not fully builded at this time.

 

[dan catalyst]

oops i said iac idle air control(domestic talk) and i ment idle speed control.it ohmed out ok.ill test the base idle w/ a light and be right back.

 

[dan catalyst]

timing belt was off a tooth on the crank.also the oil pump was 180* out.what happens when the oil pump is that far off?