I have a high milage '98 forester.  After a long period w/ very steady low
temperature readings, I was 120 miles into a trip yesterday when it started
to rain.  I was driving up a mountain in 5th gear at around 2400 rpms and I
noticed that my gauge was getting hot.  It would then cool off when I shifted
to 4th and brought the RPMs up to 3k.  This went on for the next 50 miles--
every time the car got below 2800 rpms it would go to about 3/4 the way to
red.  

So today I noticed that the coolant was low.  I took it to an auto parts
store and a guy who worked there had a Subaru said that it was likely a bad
thermostat.  I bought one and changed it. I refilled the radiator and when it
got hot enough for the fan to come on after 15 minutes or so, the temp guage
really spiked.  (BTW I could not tell for sure if the old thermostat was bad)

I tried to drive it and had to stop really quickly since it was getting close
to the red--much closer than yesterday.

It didn't exactly seem hot but I am not really sure.  

Could this occur if I didn't properly bleed the air out of the system?
Does this sound like the pump may be bad? The guage?
Any advise before I go to a mechanic that I cannot afford?

Thanks, Matt