2001 Chrysler PT Cruiser Tranny is weird--HELP

I need help. I have a 2001 PT Cruiser, automatic. This does not happen all the time but enough to worry me. When slowing down, to turn or not hit the idiot that just stopped in front of me, when I go to give it gas again, --the tranny will clunk hard. Before it does this--the speedometer goes nuts. The grounds have been checked and am told they are fine. The fluid and filter has been changed. The mechanic said the last thing to try, other than a new tranny, is to put in a new computer and see if that helps. I don't understand because it does not happen all the time. Most of the time, it drives like a dream. I don't want to keep putting money into it, if it does need a new tranny. PLEASE HELP--what on earth could be causing this?