hewey

I've tried looking through other threads but didn't see anyone posting this particular problem.

A little background on the car: it's my stepmom's car, 08 GT, owned it since new, always serviced at regular intervals at a dealership where the techs are all certified to work on Mazdaspeed variants, recently had the plugs, wires and coils changed, and it has ~90k miles.

When I'm driving up any sort of incline in 5th or 6th gear around 2k rpms, the whole car shutters. Before that would be the rpm range when I was going down the highway and it never had any problem. It doesn't happen in any other gear and it only happens in 5th when the overdrive gear engages. It's not a problem when I use manual shift mode but if I leave the car in drive, it won't downshift to get out of that range unless I really step on it which I don't remember being the case before.

Any ideas what this could be? I know that fluid is supposed to be changed at 100k so could that be the problem? I'm just worried that something is wrong with the overdrive gear.