HELP! 300SD loosing power aft 20 min driving
 

I have an 82 300SD w/ an 84 engine with a new top end. Aft. 20-30 min of driving the engine looses top speed/ power and will ultimately stall. If I shut down for 1-3 min full power is regained for another 20 min or so.
I have completed the following:
Head/ Valves/ top end all new.
Injectors good (will be upgrading to Monark)
Complete diesel purge until fluid coming out same color as in.
All new fuel filters.
I.P. Timing good. (Could be fine tuned a bit more)
ALDA Tuned, banjo bolts cleaned.
R.D. Bolt adjusted Idle smooth.
Fuel tank removed/ clean. Tank screen clean.

What am I missing? Is my I.P. Going bad? Is the fuel tank not venting and creating a vacuum cutting off the fuel flow?

Thanks for any and all suggestions!
James.

Attached pic is Olga topping out @ Buck oh five! Old engine. Used to fly!