Driving style on a used truck being different than the previous owner can indeed affect the shifting of the trans. If you want to start fresh then have a dealership reflash everything and allow for a relearn time period for the ECM/TCM to learn your normal driving style. Flushing the tranny can help, but you do (as stated) need to take it easy for a few hundred miles afterward. The main reason people have problems with a full flush at higher miles is due to the fact that the friction modifiers and other additives in transmission fluid actually adhere to the clutches in the tranny to help them grab better. When flushing with new fluid, flushing with a machine can actually clean the clutch packs as well and that can make the clutches themselves take on the properties of a brand new clean transmission. You need to give it time for the additives to adhere to the clutch packs before giving it hard throttle. One reason for the issues after a flush is due to the fact that some dealerships and shops use a chemical "cleaner" to clean out the old fluid. This not only cleans the clutches and removes the older film of friction modifiers, it can also leave a little trash in the tranny that can quickly clog the filter and restrict flow. If you have higher mileage, flushing at home with just changing the fluid and not using a machine is actually the best way to do so. No matter what you do though, once you flush it, make sure to provide a break in period just as if the tranny is brand new. I would recommend at least 500 miles to give the new fluid time to adhere to the clutch packs.