Water Heater Must be powered on and off to heat- please help

I have a Rheem PowerVent Gas water heater that when it heats, it heats very well. Lately I have found that we are running out of hot water; when I check on the water heater I find the vent fan running but the gas not lit. If I power the unit off and power it on again the vent fan starts spinning and the heater runs through its ignition cycle, the gas lights and the heater appears to heat the unit for a couple days worth of hot water. Sure enough a day or so later I have go through the whole process again.
Any ideas as to what might be going on?
Any help would be appreciated.
Jason
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quite long meybe 15 feet. I'd have to measure to be sure.

lit.
spinning
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There is a length limit to the vent runs. I'll have to know the model number to see what the max run is.

I
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