DL2 Lamping off.

I am having a problem with two of our DL2s. Four fixtures in the rig, first day all four ran eight hours, no problems. Day two after five hours, one of the fixtures would lamp off, then turn the lamp back on after a few seconds. This happened every thirty minutes or so. Swaped it with a spare unit. Today another unit is doing the same thing after working for two hours, with no more spares, all I could do it change the lamp. Any advise?