96 Buick - HELP!!!!!

1996 Buick Regal Custom, 3100 engine. Slappy #4 cylinder but it goes away when it warms up. Problem is when the car warms up, no power, hesitation during acceleration but sporatically it works fine. Have brought it to 5 different garages and 4 military mechanics but no one knows what the problem is. No codes on computer, fuel injection flush, new plugs, new wires, new coil, MAP is fine, EGR is fine. No loss of coolant(none in oil) Have had pretty well everything checked on engine. It only takes about 1-2 minutes for this to start happening. It has gradually gotten worse over the past 7 months. Anyone have any ideas?

How many miles are on this car? I have a couple of ideas especially if it is a high mileage vehicle.

could this slappy sound be a lifter stuck , until heat and lubricant is applied. sometimes they are loud, and muffled at the same time...Maybe try a good top end lubricant...Lucas?...Techron...Im note sure whats best...