I have a Rheem Criterion II furnace that recently will sometimes work fine and other times the flame will kick on and heat up, then when the blower kicks on after about 5 seconds the flame goes off and the blower keeps going. The flame kicks on about 3-4 times every minute or so and after the 4th unsuccessful try the blower finally turns off too. I was cleaning the end of what I thought was the thermocouple and it will work fine for a day. Only thing is that I just found out it is just the spark ignitor and I think it was just a coincidence that things worked good for a while as I don't believe this part would cause such a problem. Any advice on what to check next would be appreciated.

If your unit has a flame sensor clean it.

You need to clean the flame sensor. It will be a small thin rod about 2 1/2" long located by the gas orfices in the flame. Take it out and clean it with emery cloth of sand paper. They are ussally just dirty, not bad.