coolant problems

I have a 99 Sable that I'm experiencing a problem with that I need some help with. About 3-4 months ago I relpaced the coolant reservoir with an after market product. It was cracked and I figured it was just time. Since then I've been having a problem with the coolant "boiling over" and coming out the resevoir cap after the car has been shut off. The problem was intermittent at first but seems to be happening on a daily basis now. The last two mornings on the way to work - 10 miles - mostly 40+mpg driving - it has happened. Last night on the way home (much warmer, I live in Mississippi) I didn't have a problem. On a recent trip I stopped after about 60 miles and it happened - added H2O and didn't happen again. Replaced the thermostat less then 2-weeks ago. It's been about 1.5 years since complete flush was done by a service tech.

BTW - this auto has always had coolant that looks like it came out of a mud puddle. Thinking about replacing the water pump this weekend, but wanted to get a feel if I heading in the right direction.

Any help would be appreciated,
Duane

Pressure test the cooling system and the radiator cap.
Do a compression test on the motor. Make sure there are no combustion gasses getting into the cooling system.