Heater wouldn't work in 1988 Fun Maker w/Suburban furnace so I removed the unit, opened it up, and cleaned out a dirt dobber nest. Now it works, somewhat... when I switch the thermostat on, I get blower, then igniter, flame, and presto, heat! When the temp reaches the thermostat high limit, I get flame-out, then blower continues to run 52 seconds before shut-down. So far, so good.

The next time (after the temp falls below the thermostat low limit,) I get blower, igniter, no flame, and cold air for about 30 seconds before shut-down. This process repeats until I manually switch the thermostat off, then back on, then magically it'll work just fine again for the first cycle, and fail again on the next one.

Any suggestions?