why did my 518 dodge trany lose reverse and why I have to rev the engine to take off

[crappydodge]

and why does it shudder? I rebuilt my '94 518 dodge transmition it was working good for several months then I lost reverse. fluid was clean and red but brown now and now I have to rev the engine to take off and when I do it starts eather grabing or shuddering,but I never had overdrive. was told it had to do with the throttle position sensor dont now what the problem is now