I have a GTI model 4000INS about 5-6 yrs old the previous owners had it installed. What the problem is when it is turned on after a short time the flame will lift from the burner and go out along with the pilot. I can re-light the pilot and it will do the same thing again. But once in a while it will work fine for a day or two. What the manual says is #1 the high limit switch or #2 a bad thermo-generator is this something I can buy parts for and fix myself or is the manual wrong.