By the sounds of things it can be a few things. Most that have been covered. If I was to advise I would start with ruling things out and see whats left then.
1. Equipment service, reg and Cylinders
2. Gas contamination? (9 times out of 10 I would say this is it)
3. Breathing (CO2 build up from shallow breaths, maybe a smaller volume reg would also help a little.
4. Seasickness - sometimes its the victim that knows last that its that.
5. Do you use Nitrox?? If not try it out. Watch out for the MOD though and go for a mid range mix as an experiment - maybe 30-32%
After trying all this you may have the one that is causing the problem.
Hope it helps and let us all know how it goes