CIS leans warm control pressure while running???
 

We have a car 79 SC with a CIS system problem and are running out of options to verify.

We have the proper fuel pressure testing gauges and also a air/fuel ratio tailpipe monitor.

So far we have tested:
Ambient temperature about 80-90 deg
System pressure 65 psi
Cold control pressure 40 psi
Warm control pressure 45 psi
And all are within proper parameters

The problem is that once the car is running for about 10-15 minutes the warm control pressure begins to drop gradually to below 40 psi to the point of making the car stall. While this is happening our A//F ratio spikes up to 20 o more before the car stalls.

We did this test also with the fuel pump hot wired to confirm that the fuel pump was no loosing power because of the relay. With the same results.

Now here is the weird part. You turn off the key and you can start the car right back up and the pressure goes back to normal warm control pressure.

You can drive for a couple of minutes and it will start to drop again and repeat the cycle.

So far we have:

1. Installed new fuel pump
2. Installed new fuel filter
2 Removed and cleaned gas tank
3. Bench tested injectors
4. Rebuilt fuel distributor
5. New fuel pump relay

Any guidance on what could be causing this or what to test next will be really helpful.

Thanks
Ivan