Furnace Problem

I replaced the furnace in my 31' a few years ago. I have converted the A/S to a small woodworking shop. I produce a lot of sawdust. Over the years I have had continuing problems. In all cases the work was done by a Suburban repair guy. One problem was repaired with a new "board". Last year it was a high limit switch. This year the same, limit switch. I will thun the thermo to about 65 and the furnace comes on and it blows hot air as it is supposed to but only for about 15 minutes not long enough to get up to temp. the points on the thermo are still closed. If I then turn the thermo. down and then back up it starts and runs again for 15 or 20 minutes. The temp outside is in the mid 20's and eventually the A/S gets up to temp.

Any ideas would be appreciated, thanks.