Caterpillar 3406B mechanical fuel injection automatic timing advance

My cat was having a low power problem, so I removed the fuel injection pump to try to find worn out parts that could be the cause. All looked fine but now after reassembly and installation, my automatic timing advance unit is slipping and won't stay in time. I've repeatedly put the engine back in time using the timing pin method, but when the roadcheck is performed, the engine runs great at lower power settings, but slips out of time with higher power settings and the power rapidly decreases to the point of barely managing 30 mph. There are small surges of power after the apparent slippage occurs- but pull a load?- Forget IT!
Upon checking the timing with the pins, I find it's again either advanced or retarded and after resetting and roadcheck, a repeat of momentary good power is followed by slippage and low power.
The automatic timing advance unit appears in excellent shape internally-I even replaced the seals and ensured the flyweights aren't binding.
I can't be sure replacement will solve this puzzle. Any help out there?

Well, it's now known what was causing this problem. Even though the timing wasn't staying exactly where it should, the main cause of the power loss was a leaking diaphragm in the fuel ratio control. I had taken this unit apart to inspect, and could not see any pinholes in the diaphragm yet it was tested by Alban and found to be faulty. What's sad is I had a new one but didn't put it in to try out- but this was mainly because I could see the timing wasn't staying put, and was certain the advance unit was the culprit. Anyway, the bill was over $600 with about $5 worth in parts- the rest was all labor - but if one can't find the problem, what other choice is there. At least now the power is BACK!  

Did you crank on the fuel screws any, or was it set back to stock?  
.
Daily driver: 67 F100, 300, 4V, 4 spd, soon to be 4BT.
My low buck fuel rate site:
www.freewebs.com/nevrenufhp/index.htm

I have never been in favor of turning up the fuel on any engine I've owned, so the setting should be stock. I have a lack of knowledge with regard to the fuel ratio control and the dashpot settings and have chosen not to fool with these components unless I have to.
Thanks for your query!