I have a vanguard vp26ta infrared ventless heater which I have connected to propane. When it is in operation and goes into standby when temp is reached , when it reignites the right 2 placards do not lite always causing a popping and fluttering noice and blue flames burst out the face of the heater. I had the installer here, he cleaned the unit but informed me that it could do this again.. Question is" what is causing this because it is scarey situation when this happens.