I have a 175000 BTU Laars Lite propane spa heater. It is about ten years old and was working fine last year. I winterized it by draining it (and the spa) last year. This spring when I filled it up and kicked it on, it won't stay lit. When the pump comes on, it clicks, I see the glow plug light up and it lights the propane as ot always did. But it only runs for about 10 seconds, and shuts off. Then after 20-30 seconds. it does it again, and repeats this until I turn off the pump. I can't figure out why it will light but not stay lit. I thought it might be the pressure switch or the pump not putting out enough pressure, or the filter being clogged. I cleaned the filter, and it didn't help. There seems to be plenty of water flow. I put a jumper on the pressure switch to see if it would stay on if the switch stayed connected. Nothing changed. I replaced the circuit board and thermometer board a couple of years ago, and put a new valve in. So the control systems are not as old as the heater. Any suggestions?