2001 Chevy Blazer

Engine Performance problem
2001 Chevy Blazer 6 cyl Four Wheel Drive Automatic 92K miles

I own a 2001 Chevrolet Blazer 4 wheel drive 4.3 liter engine. I am also the original owner. This vehicle starts up perfectly okay and smooth. You can put a dime on the engine and it will not fall off. However, when vehicle is put into drive and I start to accelerate the vehicle has no power at all. It will barely reach 25 mph. I took the vehicle to dealer and was advised that it needed a catalytic converter. I replaced it and the problem still exists. I also replaced spark plugs, distributor cap, and rotor. The problem still exists. I replaced MAP sensors, MAF sensors, air flow sensor, and EGR valve. I also took fuel pressure readings and they checked out okay. The problem still exists. I was advised that there could be probable cause that the upper intake or lower intake manifold gaskets may be faulty. I also replaced these and the vehicle still has no pickup at all when in drive and during acceleration under load. Can anyone please advise as to what can possibly be the probable cause to this problem? As my mechanical abilities are exhausted. Please advise and help.

Thank you.