Ignition trouble

[newbe40]

1995 240sx se, I would like to ask if you know of a common problem with the internal coil distributor ?If this problem occurs is it usually intermittent or no run?The trouble I have is spark blow out at 15lbs of boost or more. I have narrowed the plug gap to .025 ngk bkr e6 this is a new problem after welding 3inch exhaust for the car.The car has run up to 20lbs before with no break up of spark.I have pressure test the plumbing and found no boost leaks.The car has a msd 6a installed with a external coil modified dis cap for high tension coil wire.

Any help would be appreciated