Zenith PRO 881 stops projecting

My zenith pro 881 stops projecting after being on for a few minutes (projector remains on). When this problem first started, it was very sporadic and it would stop projecting after approx. 30 minutes. Then it began happening more often and after being on for a shorter amount of time. Now it occurs pretty much every time I try to use it, 3 to 5 minutes after being turned on. Turning it off and then on again doesn't start the process over-- rather, it has to be off for quite a while (like turn it off and try it again tomorrow). Every once in a while, everything works fine. Any clues as to the problem?