Heatilator Caliber Gas Fireplace Problem

Alright fireplaces whizzes: I have a problem and could use some troubleshooting advice. I have a GDFL36E NAT Heatilator Caliber Direct Vent Gas Peninsula fireplace. It's an older unit that I picked up used. It was manufactured in 1999. It has an electronic ignition.

The problem with the fireplace is that it will turn on and turn off repeatedly. For example it will light, run for eight, six or one minute then switch off. It will then light back up usually in 10 to 30 seconds and repeats. There is no pattern to the amount of time the fireplace is lit and when it switches off you can here it. It sounds like the gas valve is switching off and it goes off almost instantly not gradually which makes me think it is not a vent air balance issue bur definitely something telling the gas valve to switch to closed.

I had a tech out who said he thought it may be a venting issue. I don't think he knew what he was talking about as I assume if there was an air imbalance it would be a gradual shutting off of the flames as opposed to the audible sound of the gas switch going off. However, I am just a handy homeowner so I could be wrong.

After some research, I thought the electronic ignition module was the problem. I bought a new replacement Robert Shaw DS 845 and replaced the existing. I was confident that would solve it. However, to my dismay, the exact same issue. I am stumped. Need to know what else could cause the gas valve to switch off after the fireplace is started and running fine. I can't imagine it has anything to do with air balance and venting? Is it a safety switch? HELP!

Thanks for responding. This is an electronic ignition so when you light the fireplace a little rod sparks and lights the fireplace. I don't see a pilot and don't believe this unit has a standing pilot light. Any other ideas?

I did some research and that rod is a rectifier. I bought the fireplace used and installed it myself including running the venting. It has always had the problem since I installed it. However, I think the venting I installed is spot on. I assume you were thinking that if it ran correctly for a period of time it is not a venting issue.

If it was a venting issue, would that cause the gas valve to switch off? It definitely seems like the valve shuts off which immediately causes the flames to go out. It does not seem like the flame is dying down due to the venting which in turn causes the rectifier to lose contact with the flames/cool down and shut off. Would a gas fireplace circa 1999 be advanced enough to have a system to read air/gas imbalances and then shut off the gas valve?

Also, when the flame goes out, it is always the same way. It is never a gradual petering out of the flame. It is always immediate. However, sometimes the fireplace will run for ten or twelve minutes before it happens, other times it is one or two minutes. There is no real rhyme or reason to that as far as I can tell.

Thinking my next move is to clean the rectifier with sand cloth and then check the electrical connections/ground to the rectifier rod. Anything else you suggest I try?