It sounds like what others have described as the TPS. I just changed mine and there is a little, round, plastic piece inside that was broken-like they said it would be(poor engineering). I was not having the symptoms you describe.
My problem was with setting timing-the timing would not "jump" when I pulled check engine connector-and this is one of the signs of a busted TPS. Tonkin Toyota(Portland, Oregon) quoted me 90$ but gave me it for 80$ with the throttle gasket-which you should get too if you do it with removing the throttle body too. The gasket was a PITA to remove and took me a couple days, but if I was to do it again: I'd remove the driver seat to allow more access-it'll take an extra ten minutes and save you a few awfully uncomfortable positions-then you can just put in the TPS without removing the Throttle Body(I think). Although some may say removing the Throttle body and cleaning it is necessary at some point anyway.
I read, here or TVP, that ROCKAUTO sells a TPS for cheaper and it worked but I don't think it's much cheaper and why mess around with it twice to save 20$?
Setting the TPS really wasn't as difficult as it seems so just do it right and you'll be happy.
Let us know how it goes- aaron.
Here is a link to Bala's TPS troubles and fix of the TPS:
http://www.toyotavantech.com/forum/showthread.php?57-Engine-Service-Connector-not-working-can-t-set-proper-ignition-timing.