Cjc5602

Hi guys. I've got a Raypak 266 natural gas millivolt heater on my pool that's about 5 years old. Today I went to fire her up and I've got an issue. I always remove the front panel when I start the heater up the first time of the season. Well I turned the heater on and she sparked and ignited and flames came out on both sides of the burner assembly and rolled to the front. I watched it for a bit and the heater started to shake and make noises then it shut off and then tried to restart on its own. Now I've read that there might be soot and debris on the exchanger so I removed the top vent and vacuumed the leaves out but still no luck. Any ideas on what to do next.