My Regal sometimes will not turn over (acts like.

My Regal sometimes will not turn over (acts like it's not getting any gas) if I try to start the engine without first having turned off the a/c, heater, or even vent. After about 15 to 30 minutes later and having made sure that the a/c, etc. Is turned off, it then starts just fine and runs just fine. I'm not sure if this is an engine issue or perhaps a computer issue. Note: Sometimes right after starting the car, the engine revs to about midway on the dial and then comes back down to its normal speed.