2001 Chevy Tahoe Idle Issue

Hello all, I am seeking some automobile assistance. I myself am a computer geek and know a few basic things about cars/trucks but not much. I recently
purchased a 2001 Chevy Tahoe and am having a few problems. First off, It's still under a 2 month powertrain warranty so I need to act quick. What's happening is the other night it started to rain and I was driving. I stopped at a light and the truck was idleing. I felt it shake a bit so I looked at the RPM guage and noticed it dropping and also the light's started to dim so I instantly put it in N to rev the engine in fear of it stalling and not starting back up. When I applied the gas pedal it was fine. Then I put it back in park and continued on when the light turned green. Then a few days later, I was backing out of my driveway and it did the same thing. It was not raining, I didn't have the windsheild wipers on. Just the AC and radio. I am concerned that there is something wrong here and am not sure where to turn so I took it to the service department where I purchased it. I explained to the Service Advisor the symptoms that I had witnessed and he documented them all and took my vehicle for a day. When I picked up the vehicle the Service advisor stated that they could not replicate the problem on their end and also that the computer didn't "code it" and that I have a bad tank of gas. Me being the computer geek I said ok, My wife said I am too easy. Anyway, I told the man that I used Ammco 93 Octane and he said, theres your trouble. He said that the engine is running too cool. I instantly went home and read my owners manual and I could find nothing of the sort. I feel like the man is giving me the runaround. Do you think that I should bring it back to him and Insist that they fix it or should I wait until it happens again. I have done some research and theres are the things that I could come up with.
**Major Vacume Leak **Fuel Pressure Regulator **Throttle Position Sensor **Ignition switch wires **Alternator
I was considering emailing this to the service advisor and seeing what type of responce I would get out of him. What do you guys think. Any and all information would be appreciated.
Have a super day!
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