Doesn't "sometimes" imply "not always"?

wrote:
I have an oil-burning furnace, with an intermittent problem. Most of
the time when the temperature drops below the thermostat level, it
kicks in just fine. But every so often, with no particular regularity,
it fails to start. I have to go down to the basement and press the
manual restart button (which always works with just one push). I have
found no factor that seems to explain when it fails to automatically
start. (For example, it seems no worse on particularly cold days, or
damp day, etc.) It is just that once every day or so, it fails to come
on.

I have a service contract, and the furnace is old but has been
maintained. A couple service guys have been out to look at it, and
each has done a little something, but nothing has definitively solved
the problem. (I am embarrassed to admit here that I am NOT the
handyman type. I do not watch over their shoulders, and I do not know
precisely what they have tried.)

I have a guy coming out again in a couple days, and I am hoping to be a
little bit better armed with knowledge, and be more involved in the
diagnosis. I was wondering if anyone has ideas on what sorts of things
could cause this intermittent problem.

Thanks.