This reminds me of coilpack issues. They will cause misfires at specific RPM's and will change where they misfire in the RPM range dependent upon whether the car is warm or cold. Usually when warm, misfiring gets worse. I'd suggest inspecting your coilpacks and perhaps your spark plugs. You may not see an issue though, but they could still be bad. You may have to buy a new OEM set from a place that accepts returns and try installing them to see if it solves the problem, and if it doesn't, return them and look for the next area to try replacing parts on. Do not get anything but OEM Toyota or Denso coilpacks. All aftermarket brands for the Celica are known to have issues.
I'd also suggest buying an affordable OBD2 reader, hooking it up to your car, and going for a drive with it connected. It may very well throw a code that tells you exactly what is wrong. You can get one for around $60 on Amazon.
2000 GTS 6-speed. Performance Mods: Apexi Power FC, BC Racing DR coilovers, Injen CAI, Catback exhaust, EBC 'USR' rotors, Hawk 'Street/Race' pads, Stoptech brake/clutch lines, Castrol SRF brake fluid, Exedy stage 2 clutch, Mishimoto radiator, Moroso oil pan, 11# flywheel, Braille 11.5# car battery, Energy urethane suspension/motor bushings, BFGoodrich G-force tires.