Why does my car loose power when my foot is off the gas?

That is usually a sign of the Idle Air Control Valve. If you have a throttle by wire control try to clean the throttle plates. Start your car, turn on the a/c, check your rpm to see if it stays normal or drops. A good tell-tell diagnosis.

Check engine/service engine soon light on? Let a mechanic run a scan test to find the problem is your best bet! Even if the light is not on. That's a scan test, not a code reader at a parts store!