car wont shut off?

88 gt vert. I think my ac compressor is locking up. It has only happened twice, It sounds like the motor bogs way down and a belt is slipping. When it did this I turned off the car and took out the key, it still was trying to run. I then put it in 2nd gear and let out the clutch with the brake on to stall it. The car still tried to go, it would not die. I then put the key back in and started the car ( the car was still half running). this fixed it. but it did it again. whats wrong? thanks I recently put a new ( old stock) computer in it, but it has ran fine for months.
 
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