A/C works great at idle and steady speed only

A/C was working fine until yesterday. Got in car and drove to church, no problem. When I came out to go home A/C would blow warm air out of vents unless I let off gas, was idling or was driving a steady speed on the highway. I do know that these are all high engine vacuum conditions and therefore this may indicate a vacuum leak. My question is: do the symptoms I'm describing indicate a vacuum leak and if so where should I be looking? I've checked under the hood and don't see anything obvious. I also don't hear a vacuum leak but then again I don't have the best hearing. Thanks.

What year and model Grand Prix? Do you have manual HVAC or digital climate control?

Normally with lack of sufficient engine vacuum the airflow through the vents will be improper. May only blow out of defrost and floor vents. You can test engine vacuum right on the accessory vacuum line off the intake manifold or supercharger.

Sorry, stupid me. I have a 1998 GTP with the climate control option. I will go out and check the vacuum ( if I can locate my vacuum gauge ) and post it here later. Thanks.

Sorry, stupid me. I have a 1998 GTP with the climate control option. I will go out and check the vacuum ( if I can locate my vacuum gauge ) and post it here later. Thanks.

Sounds good. Can't miss the line. Located in the middle top of supercharger. Routes around over to the battery and passenger side inner fender and into the main harness going into the firewall. Then to a vacuum distribution block under the dash located by the blower motor assembly.

I've hooked up a pressure / vacuum gauge and am getting about 15" vacuum at idle. I was looking at the vacuum "manifold" on top of the supercharger near the throttle body and was wondering, is that a check valve that all the vacuum lines attach to? I'm hooked up upstream of that point right now. I will have to check the vacuum there later, gotta go do some things now for a couple of hours.