Is the timing set correctly?

Yes the timing was supposedly checked on the tuneup by the mechanic. I just checked the codes this morning and I got codes 25 and 26 (although the mechanic claimed it read knock sensor a few weeks ago).

Maybe Oxy Sensor??

possibly a oxy sensor but it sounds kinda like a bad MAF

sounds like you should figure out what codes 25 and 26 are....and start there. Also, if the mechanic was right about the Knock sensor, you may want to look at that a little closer....like check for a frayed or broken wire very near the sensor....A knock sensor that is preventing the timing from fully advancing could cause poor power and bad mileage.....yet it will idle just fine and will rev fairly normal under no load.

edit------I could see codes 25 and 26 being thrown for a condition where an engine under load with retarded timing is wasting gas....incomplete burn....causing the oxy sensor to send a LEAN request to the ecu....

I bet if you look at the plugs they are black and sooty.
You forgot to tell us about all the backfiring it has been doing.

No backfiring at all. The symptoms remind me of a clogged cat but that's been checked at least three times!

....ok....I still have my money on retarded timing under load....possibly one way to test this is dis-co the wire from the knock sensor and drive it. It should ping. And should have much better power on the hills. But don't let it ping excessively as it might hurt a piston.

Possibly a bad EGR/modulator?

This allows the engine to effectively run lean, pinging begins, and the KS activates upon the sound and the ECU retards the timing.

The EGR slows the burn process in the combustion chamber, and without EGR gasses entering, the void is filled with air.

Did the loss of power start immediately after the tune-up? Or was it something unrelated that occurred before or after the tune-up? I am also wondering if the mechanic set the timing correctly. It's easy to forget to short the diagnostic plug. If that is not done, timing is very much off.

You did plugs and wires--how about the distributor cap and rotor?

ZUK - It definitely seems as if the timing is retarded under load. It did ping before the tuneup. I'll unplug the KS and see if it pings.

DRT - How can I test the EGR?

MN- Problem began shortly after tuneup with wires and plugs...no cap/rotor. I'll check about shorting the diag plug for the timing.

All of these suggestions seems to be leading to the same effect that I'm experiencing so I'll try one at a time and see what happens.

Thanks.

any success?