It really could be either the ISC (Idle Speed Controller) or a vacuum leak.... another possibility is the TPS IDL switch not closing properly.
To test for a vacuum you've got a couple options. What's probably the simplest is to grab a small propane torch, open the valve a little, but don't light it. Then you trace each of the vacuum lines with the torch. If you pass a leak you'll know it, the idle will shift a fair amount.
The TSRM has details on testing and calibrating the TPS (which is the second thing I'd try):
http://www.cygnusx1.net/Supra/Librar...ction=FI&P=100
Then, I'd try testing the ISC per the TSRM and if it tests out fine, disassemble it completely (take your time and be careful with the parts) and clean it out thoroughly.
http://www.cygnusx1.net/Supra/Librar...ction=FI&P=106