I know, NO ONE'S ever asked this before...be that as it may; I recently replaced my thermostat and my car is still overheating.

(sonnet's car)

Now that it's no longer sunny and warm, I find my heater isn't working so well...at least not all the time. Water pump?

Water pump sounds like a good place to start. Check to make sure that your radiator still holds it's water. If it still has all it's water/antifreeze then it's most likely the pump. If it keeps losing water, you obviously have a leak somewhere along the system.
Check for leaks first, then go for replacing the water pump next.

Good luck!

Mr(pump it up)Sensitive

Water pump it is! Nothing else is wrong, no leaks. I'll replace next weekend.