Oliver 88 Diesel Problem

We just finished rebuilding a Oliver 88RC with a Super 88 Diesel Engine. We had all the injectors redone, head work, and did a complete in frame rebuild. We replaced the Bocsh pump with a Roosa Master to correct a non-start condition we were experiencing once it got warm before the rebuild. Prior to the rebuild the tractor had operated fine until I was out running her through it's paces discing a maybe 3 acre plot. I was pulling good making good time about half throttle 5th gear. After about 3/4 of the field was done she started to have trouble and I had to shift to fourth to pull it easy. Long story short it got to the point where it wouldnt even budge it in first without stalling.
Tractor was back up at the farm and the symptoms are it will start for maybe 30 to 40 seconds running like a top then will die like you pulled the fuel shutoff. If you try to start it right after it stalls it starts very hard with a long cranking period. If you wait about ten minutes it starts up fine until it stalls like before. My first instinct told me fuel filter, I found that somehow it wasn't changed during the rebuild. I replaced it, no fix. I then cleaned and checked both sediment bowls (under tank, lift pump) and screens to find no issues. Next I removed the tank for the second time, cleaned it and installed a two inch piece of tubing in the outlet to prevent dirt from getting in to the line. All this and it didn't fix the problem. I believe it may be the lift pump now but I have heard these aren't really a common problem is this true? Also what is your guy's opinion on the situation, I hope I explained it enough. Thanks for your time.
Mike

check to see if the return line is open, all the way from the pump to the tank, it can get plugged with debris from a wearing governor ring on a Roosa which looks somewhat like coffee grounds, at this stage it is pump rebuild time

(quoted from post at 03:51:29 02/28/13) check to see if the return line is open, all the way from the pump to the tank, it can get plugged with debris from a wearing governor ring on a Roosa which looks somewhat like coffee grounds, at this stage it is pump rebuild time

Click to expand...

Already checked, sorry forgot to mention that, the return line is open with a trickle of fuel running out of it.
Thanks for your help,
Mike

Quick check for failing retainer ring plugging return fitting, when engine starts to loose power and slows down loosten timing window side cover screws to release internal pressure. If engine runs OK, pump needs repair. Have loaded sick tractors on trailer this way to get to the shop, put a can under pump to catch fuel as it leaks out of the cover.