hey ive been having some problems with my auto.

it started with the occacional simple shudder on take off. then it would constantly slip and rev its arse off whenever id accelerate. I checked the fluid and it was filthy so I gave it a service today. got about 5Lts out of it and changed the filter and gasket. drove it for about 10mins and it seemed a little better but not 100% and then it decided it didn't like 3rd gear and now it wont change up from 2nd. the fluid is filthy again.
I first noticed it not too long after driving through a lot of water and mud at work one day.

any suggestions?