My gas wall heater will sometimes not ignite. The pilot light is lit and when I turn on the heat I hear the themostat engage. However, the heater doesn't always fire up. When it does, sometimes it turns off prematurely after a few minutes. I don't want to spend $100 on a house call if it's a simple fix. Further, the problem seems to happen on 2 of my 3 heaters. Please help!

Is this is an unvented gas heater? You didn't provide much detail.

Assuming it is, I have seen that happen when a spider took up residence in the gas tube just before the burner. Unless you are confident of your abilities, get a professional. These are not exactly intuitive when reassembling. If they aren't put together CORRECTLY it could cost you much more than a furnace repair bill.

Have you tried bypassing the thermostat, to see if operation is sustained?