Suburban Furnace, drive me to drink!

Two yrs ago my furnace would not fire off, It did everything
except ignite the propane. Cycles the fan for 8/10 sec.
then goes into the 3 step to fire off hte propane. I could
smell propane.
At that time the net said change the igniter or the board.
Cheepest part fist, changed the igniter, worked fine in the
summer. Why does the heater brake in the winter and works
in the summer?.....

Long story it would not ignite in FL. last yr. Same thig
everything works but will not ignite the propane...

Now in the process of checking everything AGAIN.....
Pulled up the floor vent no obstructins as I can see,
checked the spark igniter its still 1/8 inch off the
burner and everything looks clean and correct.

I had to finally restrict the exhaust on the furnace while
the igniter was clicking and after a couple times ....
WE HAVE IGNITION...Let it run for 30 min or so.
The second time it did not fire till I restricted the exaust
again. Ran it for maybe 20 min again.
NOW it has started twice in a row for maybe 10min at a time

I am thinking a piece of trash in the propane orface, sort
of restricting the flow, or hell I just dont know.
All voltages are good, and everything looks good. Gas stove
and gas water heater all work great.

I did notice something I did not think was right,all thou
temp was 80 deg. today. The furnace would cycle off and on
much like the over temp sw. would turn the propane off and
on. This was all while the fan continued to run and I
suppose the chamber was hot enough to ignite the propane
again.........

Let it set for an hr. and it fired up again, will try
to fire it off again tomorrow...Any input would be
appreciated, thanks in advance for any help...TENN