Is my clutch slippin or am i trippin?

[5.0PiMpin]

hey guys wats up? for the past week or so when i have been driven my 89 gt, in third and fourth gear when i reach about 4000 rpm my car feels like its stalling just a little. i just added the 3.73 gears to my ride and advanced the timing to about 131/2 degrees. i also run 92 octane 76 gasoline. i just got a new center force stage 1 clutch put in (last march) i really hope its not the clutch , i heard that they dont last as long as a stock clutch , if you arent driving the stock one like a mad man that is. when i say its stalling its more along the lines of hestitating just a little. if any body can give me some advice or suggestion to what the real problem could be i would really apericiate it. i heard that advancing the timing causes "detonation". can that be the problem.. thanks guys

PS:should i use a cooler thermostat since i am running around 14 degrees timing? thanks