No power or acceleration for first 5 miles of driving

96 Corsica V6 3.1L engine 175,000 miles.

This has been a great car for me with few problems. On start up car idles and starts down the road fine. When I get to about 30 mph or so, it's like I'm pulling a house behind me. Car has no power and the more I push the gas pedal the less it wants to go. This continues for about 5 miles of driving then it seems to correct itself and accelerates to normal driving speed. It runs fine after the first few miles with no addition problems.

I have try letting the car idle for 10 minutes to get the temperture up to normal and this has not helped. This problem comes and goes but is happening more often now.

I have replaced the fuel and air filters, PVC Valve, fuel pump, air intake temp sensor and cleaned the Map sensor with the proper cleaner. There are no codes present now, but I have had some in the pass that say lean fuel mixture. I have had it to the local repair shop and they couldn't find the problen either.

Any help would be appreciated greatly. This is the car my daughter was driving but I can't let her use it in this shape. Thanks!