Idle issue - drops rpm but recovers

[Mybestsupra2]

Hi, there are many items that need to be checked for a situation like yours. My feeling would be that the fuel reg is not the problem, as it runs fine down the road. But you could check the fuel pressure following the TSRM before wasting money on parts.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/
If you are sure that there are no vac leaks, and the timing and advance are spot on, the next best place to check is the AFM (if a Turbo it's a Karman Vortec System, and do not try to clean it with anything). Check it using the TSRM specs. The next thing to check is the TPS. Using the TSRM specs make sure it is in spec. And then the IAC, mostly that get very dirty and a good cleaning will get it working right again. My best guess is that your problem will be in one of these areas. One other item that sometimes makes a difference is the Dashpot. It allows the idle to come down slowly when coming off the throttle.
Luck