2002 GMC Envoy Car is stalling at times - What could it be.

This morning I woke up thinking that my car will start up as usually. However, the car wouldn't start properly. All the lights were on, the dashboard lights were on, and head lights came on but the car wouldn't start. I was able to get the car jump and everything came back up. I drove to get my battery tested and alternator tested - Everything came up "Good". I notice that my RPM is not running full charge. When the car started, the performance dropped down. When I came to a stop sign, my call stalled as if it was going to cut off but I mashed the gas and it went into the normal driving condition.

I don't know wheter I need to get a tune up. I haven't had one since. What other suggestions could it be? Someone told me that being it's cold, that may have caused my car not to start up.