kene

iTrader: (2)

an old rotor and cap is not likely the problem. That usually leads to misfiring during idle.

hmm... are there any obstructions in the intake tubing? what condition is the air filter? Is it clogged?

On quite a few toyota's from the 90's and later there is an idle adjustment screw[usually recessed and a flat head type] on the throttle body.(is a bit late now for me to check on my gs..I think it might have an ISC valve in its place. If so then this may be bad and need to be replaced)
If there, did you touch or adjust this screw at any time?

Also you may want to make sure your IAC(Idle Air Control Valve...helps assist in the control of air in the vacuum system by letting in more air to prevent a stall) is plugged in, or at least working fine(if not, check the junk yards..its ~400$ online)
Locate it and with the engine running unplug it and see if there is a difference in idle(if so then it is working, otherwise it may be bad)

let us know what you find.

Glock20C

$400!!! Hope its not that then. I will check it out and reply later. Thanks for the info.