I would check the tranny fluid. Any tranny maintenance in the last 12 months? Those are the same symptoms that mine displayed. I had it flushed at an Isuzu dealership, I believe they did not follow the correct procedure for the refill. About 5K miles, 1 yr later, after the tranny running hot, hot enough that I could feel it at the Ebrake handle, it would thunk into 1st and second, then it started slipping 3rd and 4th. Next thing you know, I bought a new tranny. I think the current one is going to have the same problem, I printed out the instructions for how to fill our transmission, when the guy turned it back over, the papers were spotless clean. I doubt he even looked at em. I have noticed delayed shifting, a hesitation into third, but not hot yet. Glad I got 3 yr warranty.
I would service it yourself, ASAP!!!!