Dreaded Multiple misfire... Ideas?

[ray244190]

Here we go....
P1341, P1255, P1149, P1353 with negative vacuum pressure when pushing on gas pedal
So far I've changed fuel filter, plugs, coil packs, air filter, checked intake gaskets, sealed with red glue on second pulling, oil separator return hose, vacuum hoses on intake, and oil return, also replaced line to go to the fuel regulator (to where it goes into the firewall. Tested gas pressure... 50+lbs in a few seconds.
Found a small air leak in the intake using brake cleaner, so I pulled the intake and cleaned everything really good (IT NEEDED IT!!) Cleaned the injectors while I had the rail off also. Had to raise the intake awhile back because of split tubes to rear manifold.
Everything I've done has needed to be done with 145k on her. I've tried everything I can to find out why there's no positive vacuum, well I might get 6lbs every once in awhile.
To beat it all i lose all power at 3,000 rpm!! She starts to buck and backfire. Does ok below 3 grand first through 4th, and then she is ok when she gets to about 2,500 in 5th gear. About half horsepower at her best moments.
I am not going to start throwing parts at her to find out what's wrong. I've tried to find any additional air leaks, and can't find a one.
I would be more than grateful for any ideas to find out what is going on. I also had a upper 02 sensor on bank 2, so I swapped them over to see if there was any change... NOPE... I am out of ideas to figure this out, and getting ready to move, so don't want to dish out all the extra money to a dealership 2 hours away so they can tell me that I need new plugs and coil packs because that's what the codes show.
I do use the Torque app. 25% throttle is about all I can give her.
I do have a nice pull of vacuum off the back of the intake and I don't have any issue with the brakes.