In my experience, if an automatic has too much fluid, then you can
wind up with bubbles in the fluid. This in not good for the trans at
all. So I would say too much fluid.

However, if you have not added fluid lately, then it seems unlikely.

Since you have 262k km on it, when was the last time it was serviced
(fluid & filter)? If not recently, then you might want to take it to a
reliable shop and get that done. This would resolve the issue.

Note that the 3 speed transmisisons are much more reliable than the 4
speed transmissions. You have my sympathy about the 3.0 litre
Mitsubishi engine though. I used to have a 94 Acclaim with that engine
and the 4 speed trans. Man what a bad combo. Nothing but problems,
problems, problems.

hth