My first impression is that if it revs fine with no load, but then breaks up with load.. it would mean that you have a weak link somewhere. Again, I would think it's a weak link, and not an actual "something is bad" problem - if that makes sense.
When did this start happening with your car? What did you change right before it happened? I would bet my money it has something to do with the fact you messed with the timing. It needs to be on the correct tooth at the correct degree.
I know some people have recommended things, here's a list (in order) of things I'd try if this were me:
1) Check your codes. This should ALWAYS be first when diagnosing any problems with your car. (
click here)
2) Check/verify your timing to be 10 deg BTDC, with T1 and E1 jumpered in your diagnostic box, with the engine warmed up (
click here)
3) Check your grounds on your coil packs, there should be a ground going to the head. Maybe try the "igniter ground mod". (search for it)
4) Replace your spark plugs. It's cheap enough, can't hurt, and may fix your issue. Your old ones may not be gapped correctly. Make sure they are gapped correctly, check the TSRM for gap information.
Bookmark this link:
http://www.cygnusx1.net/Supra/
It's got links to the TSRM (repair manual), TEWD (electrical diagrams), EPC (part catalog), and how-to guides by the owner.