These are the kinds of problems that cause doctors to run a bunch of tests, then shrug their shoulders. Go to a doctor anyway, just don't get your hopes up. I doubt the problem is totally random, so try to figure out if there is a trigger. Here's a thought: go buy some AZO (OTC urinary tract pain killer), and next time it happens take one quick with a glass of water. If the feeling improves before your next piss, the problem may be at the surface of the bladder wall. If the feeling improves shortly after your next piss (which will be very orange), then the problem may be at the surface of the urethral wall.