914/6 3.6 Conv. w/a voltage drop

[9146INSC]

fellas,

I really need some insight on a continuing electrical frustration.
I have a '70 914/6 w/a 1995 3.6l conversion.
Original wiring harness has a PMS plug in adapter wiring harness to make everything work. No cutting and splicing was done to put any of this car together.
Now the problem. Every time the car is started it cranks and runs fine for the first 2-3 minutes or so, then the problem begins. The fuel pressure begins to fluctuate more and more til i is swinging from 25-45 psi.
In trying to diagnose the problem here is everything I've changed and the order I've done it.
1. Replaced the fuel pump.
2. Replaced all the fuel filters.
3. Replaced the fuel pressure regulator.
4.Checked line voltage to the fuel pump with car running. I found it to be 13.5-13.75volts at start up, dwindling down to 10-10.5 the longer the car ran.

5.I then ran the car with the fuel pump straight wired to the battery post. Same problem.
6. I pulled the alternator to check, found I had to replace the engine wiring harness.(infamous '95 993 wiring harness recall issue).
7. Replaced the engine wiring harness.
8. I had the alternator checked & repaired.Pumps out 13.75v & 115a on the test machine with a load.
9. got everything put back together this weekend and with the car running a good 10 minutes the electrical system is reading 12.3-12.4v at my cutoff switch.
I know it should be 13.5-14v minimum. I've put a meter on every other car I own, and that is where it reads.
The last thing I'm gonna do is run the fuel pump off a battery sitting on the floor next to the car to isolate it.
right now the car is only good for moving around the yard. To much voltage fluctuation for the car to be dependable.
Any help would be appreciated. What have I not tried yet??

Ron

[Joe Bob]

Load test the battery?