We have a 99 Tahoe and when we get it going above 2500 rpm. (mainly when it is going up a long hill)
it as chattering feel and the check engine light comes and power lowers. The check engine goes out after a few new startups. Any ideas on what it could be?

 

Sounds like possible detonation or misfire, sometimes this can by caused by a lean condition.

I would look into having the fuel injectors cleaned thoroughly and go from there, I recommend buying a four pack of 16oz bottles of Chevron with Techron Fuel Injector Cleaner form Sam's Club or Costco (under $10), add all 4 bottles to a full tank of fuel and drive till the tank is empty.

Also make sure that the fuel filter is good. If the Tahoe has over 100,000 miles then install a new O2 Sensor.

If none of these cure the problem then have a dealer hook it up to the computer to find the problem, could be a faulty injector or spark plug or who knows what.

 

Thanks, I'll give it a try.