Two things here...
When I first bought my Mystique (God how the years go by) it had worn plugs and wires and I had sorta the same problem. Only difference was the fact that it happened so fast, you had no time to actually take your foot off the gas or press it. It would just BANG at highway speeds, with no pattern at all. Changed the plugs and wires and that took care of that.
If it is a tranny issue, then I would go for the MLPS first. It is right on top of the tranny. If it's a V6, then you will have to look under a tight knit of hoses to see it, but you will find it under the thermostat housing. On the 4cyl models, it is right there in your face. It will have a wide connector on it, so it is not hard to find.
I would also try a fluid change too. You just can't beat new ATF in a transmission.
