where should I begin troubleshooting?

Hello, it is difficult to tell if you have an engine performance problem that exists at certain torque ranges and low engine speeds such as after an upshift -- or if you have an automatic transmission concern. This will take an experienced technician to have a look at your vehicle and perform a road test to see what the vehicle appears to be doing, besides also having diagnostic equipment hooked up to the OBD II port and able to monitor a data stream of various components to see where the concerns lie.