My Tahoe when under load and accelerating would Loose power and backfire, knock, and ping this happened for years and every time I took it into the shop to get it fixed nothing was found to be wrong. So one day I was diagnosing another problem a rough Idle a government mechanic suggested that I get the ECU tested at auto Zone (for free). It Failed but the truck still worked. It was explained to me that it would still work but it would bleed voltage and cause the Idle to be rough and possibly cause misfires. it cost me $75.00 to replace the ECU and the truck has been working great I have not tested it under load yet but as far as it is running now I'm sure that the other problem is gone as well. good luck with yours.

When pulling a heavy load uphill the 99 Tahoe loses power until you are in 1st gear and you must keep your foot on it. After the top you can only drive in lower gears with your foot feathering the gas.Once you reach the top of the hill you can turn off the truck and wait a few minutes and restart, then it runs fine until the next hill. When you turn off the truck it sounds like a flap either opens or closes under the hood, then it runs fine with no codes. If you have no load it runs fine uphill all day.The dealer in Casper WY checked codes and it had a MAP,Cat code. They checked the exhaust and found no problems or back pressure. They said fuel pressure was on the low limit(55psi) so they replaced the pump and filter($784 later) the problem is the same. The codes do not come back until you pull a hill with a load.