2001 Malibu surging

My 2001 3.1 Liter 6 Cyl Sequential-Port F. I. Malibu with 128,000 miles on it runs fine. Until it starts uphill or has any load whatsoever on it. Doesn't matter if I'm in Cruise Control or not, it begins surging. I can 'punch' it and it instantly stops the surging. If I just gradually give it gas or it is in Cruise Control and has to automatically speed up it surges and is getting worse. I have had it to the shop many times and no one can figure it out. REPAIRS DONE SO FAR: plugs, plug wire, 2 air filters, ignition cables, ignition timing and idle speed adjusted, PCV valve replaced, cleaned throttle body, mass air flow sensor, injectors, linkage and all related parts (used a fuel system cleaner, and this has been done MANY times), replaced the in-tank fuel pump (included module E3507M), and fuel pump filter, replaced oil pressure switch with light and push in retainer.
I have my oil changed and serviced every 3,000 miles. Had fuel filter replaced again last oil change, still didn't fix the problem.
Please, can you tell me what to do?
Thank you.