Sporadic Heat, and Pegged Temp Gauge

[nuarctable]

Hi all.
My wife drives a 98 Astro van, with 185K miles.
She called me this morning while we were both on our way to work with a problem.
After driving about 15 miles (5 on side streets, and 10 on the highway) to where she works, she slowed down at the stoplight. At which point the heat started blowing cold, and she noticed the temp gauge was pegged.
The temp gauge was pegged for maybe 30 seconds, and then it started jumping all around. After the light turned green, she took off, the gauge went to normal, and the heat started blowing hot again.

About a year ago, I replaced the waterpump that was leaking, the last time she had a problem similar to this, and it worked fine then.
I havent checked the coolant level, but she knows enough to check that, so I dont think its that. But I havent been home to look at it yet.

Could it be a blocked heater core, or air in the lines?

Its starting to get to the cold part of the Chicago winter, and if I cant get it fixed, I will be driveing her van to work while she drives my car...

Any ideas to look at or check?
Thanks

[brcidd]

Sounds like you are low on coolant- this is most common failure mode of no heat complaint-- so check it- if low- look for the leak- water pump is first- then hoses and radiator of course- don't let that puppy run hot- not good on your engine.....