Fuel pressure issue

[rickpaw]

So I just completed a LM7/4L60e swap in my 67 Firebird. I had about 300 miles on the car after the swap, mostly around town with 30-40 minute trips. Last week I took it to Cruisin the Coast (CTC) from Houston and ran into fuel pressure (FP) issues 4 times.

Here's my fuel set up:
-LM7/4l60e combo. Tank is Tanks Inc FI tank with Aeromotive 340 pump. Corvette fuel filter/regulator mounted just in front of the tank. Brian Fich duel vent tank vent, along with stock vented fuel cap.
-Fuel line is -6AN from tank to stock 3/8 hard line on the frame rail. 6AN from pass side rail in front of firewall looping along the top of firewall to the driver side fuel rail inlet.
-Stock LS1 intake/fuel rail/injector. Stock LS1 ECM with mail order tune from PSI Conversion.
-Voltage at the pump: 13.5V. Alternator puts out 14V. Pump is triggered by ECU and powered thru 30A relay directly from the battery in the trunk. 10awg pos/neg wires.

The issue:
-The first 250 miles of the trip the car ran great. The last leg of the trip (Houston to CTC), the weather got warm so we had the AC on. After about 2 hrs driving with AC on, the car started stumbling and lost power. Pulled over and the car barely idled. I hooked up FP gauge and it only showed 20 psi. Took the pump out and found some debris blocking about 50% of the screen at the pump inlet. Cleaned it out and put everything back together. Got 40 psi of FP (voltage at pump=13.4V). Not enough but enough to get the car running to CTC (we were 30 mins away). During CTC it died 2 more times with FP of 20 psi. All while AC was on and in stop and go traffic. Both instances we took the pump out and the inlet was clean. The fuel filter/regulator was removed and checked for debris. It was clean also.
-We ordered/overnighted a 2nd Aeromotive 340 pump and changed it out on Sat, figured the 1st one was bad. Drove the car around Sat with no issues (AC was not on). Drove back to Houston from CTC Sunday. About 160 miles into the trip (after the AC had been on for 1.5 hrs), the car stumbled and died again. FP was at 20 psi. Let the car sat for around 15 mins, and FP jumped up to 40 psi. One of the old school guys in our group mentioned the engine sounded like it was struggling for air. We removed the air cleaner and it ran better so we cut a hole in the K&N filter to allow more air in. We drove the car back to Houston without the AC the rest of the way without any issues.
-Let the car sit over night and start the car. FP shows steady 58 psi.

I ran the car in the driveway yesterday with the AC on full blast, weighted the gas pedal down so the engine revved to 2500 rpm (cruise rpm when it happened). Obviously there's no load on the engine since the car was just sitting. Ran the car for 30 mins with the hood closed. Got it pretty warm to the point of the coolant overflow was puking, temp got to 242. Everything in the engine compartment was hot, but the fuel pressure showed a steady 58 psi. I think that's just ruled out vapor lock.

Any ideas/tips would be appreciated. Also during the trip, I had intermittent code of "knock sensor bank 1 low voltage input" showing up.