I cleaned out the throttle on my 90 GXE a few days ago. I just unbolted the front casting from the throttle body and sprayed/brushed it out. The front casting, which connects to the air intake duct, looks like it has 2 o-rings in it to seal against the throttle body. They aren't. Nissan uses a liquid sealant to seal the parts. I put a very thin coat of silicone to re-seal when putting it back together. Without the sealant there was a small air leak and the idle speed was off. After cleaning the idle speed may be off anyway (high) and need to be adjusted.
Cleaning the throttle body made the biggest difference in eliminating the idle dip (engine is idling, suddenly drops a couple of hundred RPM, almost stalls and then returns to normal idle speed). This was after replacing first the spark plugs, air filter and then ignition wires (NGK).

 

i just recently cleaned out my throttle body.. and my intake manifold... it took me most of the day.. and there was a lot of carbon build up.. but.. i didnt reseal the throttle body , where there it looks like it had two o-rings... um.. may be i did.. i forgot.. well it runs smoother..and stuff just wondering what cleaning out all that gunk does anyways??

 

Yea I plan on cleaning mine out since it does the same thing you say your 90gxe does. idles then suddenly drops rpms and almost dies sometimes does.