I have an older Lennox G20 system that I'm having trouble with. When a call for heat arrives, the pilot light lights, the main burner comes on and after 30 seconds the blower motor comes on. However, within 2 or 3 minutes the system will shutdown and go through a restart. I assumed the problem was similar to other systems like this and it had an older style ignition system, so I replaced both the flame sensor and the ignition control board. The problem is still the same, however now I have an LED to help troubleshoot. The led flashes 8 times which doesn't match up to any of the diagnostic codes in the manual. After further testing, the board is grounded properly, the wire to the flame sensor ohms out good and the connections are tight. 24V line has 24.5 volts and holds steady until shutdown. So, I'm kinda at a loss. Any suggestions?