I had the same problem after my install with the spike drop and was like WTF is going on. So me and one of my boys looked at the car, make sure everything is zip-tied around the fuel pressure regulator and and check you UICP hoses, under such high boost they are felxing alot and could be slipping. I replaced the stock clamps with racing clamps and problem solved, boost holds hard.

 

So ti could be that the STOCK ic piping is not holding the boost?