I have a Rudd Gas furnace that has started giving the following problem: When it starts up sometimes the burner lights then shuts off after only a few seconds. It will try 3 times to relight but does not stay burning. The blower motor comes on even though the burner is off and will run for several minutes then it too shuts off. When it does this I must turn the thermostat off and wait a few minutes for the system to reset before running it again. It always starts and runs fine after I restart it.
I am using a programmable thermostat that changes when the timer tells it to, either raising the temp or at night lowering it.

This is not something that is consistant, it never happens under the same conditons or during any particular timer cycle.
Any Ideas????