There isn't that much back pressure that would cause it to back up and release the air in spurts ~ I've never heard of anything like that happening to any diffuser. Plus, you said that the bubble pattern never changes during the revs up and down, correct? I think it is a compressor/voltage issue. Be careful shutting those vavles too much ~ you don't want to run the PSI up on the vane compressor. That is why the pop off valve keeps dumping air.
