Some possibilities:

1. the plastic end cover is cracked. When the room heats up the crack expands allowing the air pressure to fluctuate.
2. the air pump pressure is not correct. (instructions at www.reddyparts.com )
3. the fuel filter is restricted.
4. the nozzle is restricted or worn.
5. there is an air leak between the pump & the nozzle.
6. water in the fuel. When a droplet passes through there is a momentary flameout condition which will cause the photocell to shut the unit down.
7. The photocell is defective