Plus two on checking codes. You don't say where you're located, but if you're not in CA you can go by autozone (and some other parts stores) and they will check codes for free. Guessing at things and trying different solutions (as it sounds you've done) is just frustrating, time consuming and expensive.

Of course I typically recommend that everyone who's interested in their car should have a basic code scanner, the MS309 is a nice basic standalone OBD-II compliant tool and is about $20 on Amazon.

If it is the Throttle Position Sensor it's relatively inexpensive and easy to change.

In the meanwhile I'd certainly try the adaptive throttle/shift reset procedure (unique to the five-speed transmission), it's easy and free and it won't do any harm.

Good luck.