Transmission slipping

I've got a 1990 Escort and the transmission is slipping. It slips bad when I first drive it, then after the car runs for a while(30 minutes) it gets better. I've had the fluid level checked and it's ok.
Any ideas what to look for before I take it to a shop?
The car's also due for an oil change.
I've only had the car for 4 months so I can't tell you it's history.

Thanx

Is there water in your oil? If so, you're screwed- get a new tranny. If not, get it fixed.