Truck Repair: 1996 FL-70 NO POWER, bellhousing, fuel transfer

Question
HEY CLIFF....FIRST LET ME COMMEND YOU FOR PUTTING A LOT OF YOUR TIME IN THIS....THANK YOU

I HAVE A 96 FREIGHTLINER FL-70 6 SPEED....WHICH I BELEIVE HAS A 5.9 CUMMINS (THE I.D. PLATE IS MISSING)
EARLY THIS SPRING I LOST ALMOST ALL POWER..I CHANGED THE FILTER AND IT WAS A FIX....SINCE ABOUT AUGUST I'VE NOTICED A DECREASE IN POWER. IT SEEMS TO BE ONLY IN THE HIGHER GEARS. IVE REPLACED THE FILTER ABOUT ONCE EVERY TWO MONTH SINCE THEN, BUT NO CHANGE.  NOW WHEN IM ABOUT TO SHIFT TO 4TH OR 5TH THE ENGINE BOGS DOWN OR CUTS OUT.  6TH GEAR I CAN ONLY GO 45-50 MPH.  ONCE UP TO 55 THEN IT FELT LIKE A GOVERNOR KICKED IN - ENGINE HESITATED FOR A SECOND OR TWO THEN PICKED BACK UP TO AROUND 55 THEN HESITATED AGAIN.
 ALSO MY TACHOMETER EVERYONCE IN A WHILE SEEMS TO BECOME ERRATIC JUMPING ABOUT 5000 RPM'S - WITH NO CHANGE IN ENGINE NOISE OR SPEED- THEN IT WILL DROP BACK DOWN TO NORMAL...
   I COULD WRITE A BOOK ABOUT THE SYMTOMS THE TRUCK IS HAVING....I THINK I JUST DID!!!

HUMBELY REQUESTING HELP,   MATT

Answer
I would check the wastegate bellows on the turbo,fitting("T") on the fuel pump for wastegate/AFC,& fuel transfer pump pressure(Should be around 20 PSI just using the primer button).That tach is off the ring gear in the bellhousing probably on the top drivers side.