Something like this happening to mine. It works but is not as cold as i wanted it to be. Ac kicks in when i push the button, rpms up but its not like freazing cold. And on colder days it works better. Plus the air flow spreads in all vents at same time, feet,windows and in the face.
I would "GUESS" that your refrigerant level is low as well... there is still enough in there to work when it doesnt have to cool that much, but not when you really need it. Charge it up and see what happens.
98 GTP Coupe- 180* T-stat, GMPP Front Sway Bar, Dual Aeroforce Interceptor's, TDC FWI, TDC Taylor 10.4's, ZZP SS Plog, ZZP 3" SS catted DP, ZZP MPS W/3.4, AL104's gapped @.055, 3800Perf. PCM