There's a piece inside the transmission that sometimes slips out of place, and it can cause that, also, there is a small chain in there that could have broken.
We've addressed this, with some useful advice, in another thread, as well.
Scroll down to the bottom and click "Older Messages", then look for the Thread titled "head gasket" which will be near the bottom of that screen.
That dude had a similar problem, and you may wish to e-mail him and see how he handled it, and possibly find out if you have the same problem.
Let us know if ya need more help than that.