I have a 91 Bonneville SSE and while I was driving with the A/C on it automatically switch from blowing out the vents to blowing out the defrost. I hit a bump in the road and it switched back to vents. It switches at random times and with it blowing out the defrost it doesn't cool as well. If I hit the dash hard with my hand it will switch back to vents. Does anybody know what the fix is for this problem? (My hand is getting sore.)

mine does the same, if anyone has a answer to this i would greatly appreciate it aswell.

First check the vac lines under the hood, they should activate a damper-door that switches exhaust flow location. On my '88 the hose had just popped off (near the firewall between middle and driver'* side if I remember correctly). Might be a one-way valve there too which could be stuck, which couples a couple different lines into one.

I checked the vac lines under the hood and everything is connected. I sprayed some WD-40 in the one-way valve. I hope that takes care of the problem.

replace vacuum lines. check vaccum lines behind the ac cluster if u want.