Jay,

If your fan cycles and not the burner, it sounds like the air flow is
set too high. Not a limit problem at all. The duct cleaner is
clueless. It may be that the blower shutoff or turn on settings are
too low or out of wack. Try settint the On at 135 and Off at 90 to
100. If it still cycles, set the heating blower speed lower.

If it is 60 to 90 minutes between cycles, even when it is cold outside,
your furnace is oversized. It should run almost constantly when at
design conditions for your area.

Stretch