Could be anything.

One of the fans might not be turning. The projectors I work on here have3-4 fans, and they all have tachometers (a third wire). some times the tac goes bad and the fan has to be replaced. The only way to tell is to use an oscilliscope to measure the signal from the fan, or to replace the fans one at a time. If that doesn't fix the unit, then you might have a bad: power, main or ballast board. Again, the only real way to find out which is by measuring or substituting.