Rapid RPM drops and stuck at 2k RPM

Hi all. My RSX has two CEL one is code 0505 for idle control system malfunction, and the other is 0128 for thermostat issue. My idle is always consistant and i never have issues with it but if im idle for too long, next time i try to take off in first gear, it will act like a 2 step and not let me go past 2k rpm. So if i take off at 1500 rpm and try to shift to 2nd gear at 3k rpm itll just automatically drop to 1k as soon as i put the clutch in. almost impossible to shift without revving the car before putting it into gear. for the 0505 code I got to the iacv and cleaned it really well along with the TB, but the light is still on and the problems still happen. I haven't gotten around to changing my thermostat yet but im gonna do that soon. Would changing that make all this go away? or has anyone had this happen to them? Any help would be appreciated +1

Rapid RPM drops and stuck at 2k RPM