I had this a few years ago - it came and went, and eventually I discovered that it was caused by:
changing my normal diet
being under stress
doing more exercise than normal
and having way too much caffeine
All of which tended to happen at the same time when I was away for work. I had blood tests and all sorts -the doctor was also wondering about diabetes - but there was nothing found. And then it stopped - mainly because I changed what I did at work I think.
Can you trace any pattern in yours, or does it seem to be more random?