Shifting problem

I've got a 2005 Rendezvous and about a month ago it started shifting hard at every shift. I'll hear a thunk and feel a bit of a lurch as it shifts. It doesn't happen every time I drive it though. It was happening on Monday but ran smooth the past three days. Could it be some sort of electrical/computer problem instead of a bad tranny?

If its a mechanical problem, I hear Lucas transmission fix might help. For more details, surf: http://lucasoil.hodgsoncorp.com/slipping_transmission.htm

If an electronic problem or a mechanical problem that doesn't touch the transmission fluid, then no "additive fix" treatment will work. It will need to be fixed by a professional mechanic.

Hope this helps...

.