general car question

Currently reading:
general car question

Might sound daft, but a good "blat" may do the car the world of good, take it for a good long run and make sure it's warmed all the way through.
From your post, it sounds like it's not getting warmed up enough and while it may be that the thermostat needs to be changed it's more likely the car is (for want of a better expression) a bit bunged up.
Short runs, not fully warmed through = what you describe in my experience :(