Thanks again for all your help. To recap the problem: it sometimes would take 6
hours for house to heat up 6 degrees, and occasionally the house would be colder
than the lowest programmed thermostat setting. The problem was compounded because
it was intermittent-- sometimes no problems, sometimes problems.

As a first fix, I replaced the batteries in my thermostat. Turns out they were low
on juice, but not so low that the "lo batt" indicator came on. The first two days
after I changed the batteries I had no problems. Then (on a weekend, of course),
the house took 6 hours to heat up, with the furnace blower staying on for at most
a minute or so at a time.

The repair guy came on Monday. I have a Trane furnace, about 12 years old,
everything original. Repairman turns the thermostat from 70 to 75 degrees. Of
course, the furnace works like a charm, heating up the house a degree every 10
minutes or so. Furnace guy says it's the dreaded intermittent problem, which is
hard to diagnose. He suggests 4 possible causes:
1) Dirty flame sensor (we had not had ours cleaned in a few years)
2) Problem with condensate pump. He said the flow of condensate from the furnace
was very slow, and possibly there was a clog there that caused the furnace to shut
off.
3) Bad thermostat. He thought this was not likely, but possible.
4) Bad control panel.

He suggested that he try to fix (1) and (2), since they were likely culprits and
not too expensive to fix (unlike the control panel). Fortunately, the furnace shut
down when the house was only at 74 degrees, and he said it seemed like some kind
of pressure problem (over my head-- I had no idea how complex the furnace was).
Anyway, he cleaned the sensor and flushed the pump, and everything has been
working well the past few days. Here's hoping that was all it was.

Thanks again, all! If I have any more problems, I'll post.

Doug