I have a Pyromaster vent free gas fireplace. It is only 2 1/2 years old. It worked fine when we took it out of our old house. It sat for about 7 months while we had an addition put on our new house. Now when you turn it on it will burn for a while, shut off and then usually starts back up. Sometimes it doesn't start back up. If you turn it off after it shuts off and then back on it will usually relight. The pilot never goes out. It has 2 rows of flame, occasionally the front row won't light, but after a few seconds it will usually then light. It operates off a millivolt gas valve. Is my gas valve going bad or could it be the unit that sits in the pilot light? I tried vacuuming out the gas tubes. Any advice or troubleshooting tips would be appreciated. I am fairly knowledgable when it comes to home repairs so any suggestions as to things I can check would be great.