Rough Shifting into Third and Overdrive

I have had this problem for some time on our 2000 Century and posted once last year. Dealer says it is dirt on the plates and to far up to get to it with a pressure wash. He said the problem will go away after driving for some time. It Does!

Does it get better because the circulating transmission fluid washes off the dirt? If this is the case, why can't one drive the car until the problem goes away and then drain the fluid with the dirt and refill with new fluid?

I have had this problem for some time on our 2000 Century and posted once last year. Dealer says it is dirt on the plates and to far up to get to it with a pressure wash. He said the problem will go away after driving for some time. It Does!

Does it get better because the circulating transmission fluid washes off the dirt? If this is the case, why can't one drive the car until the problem goes away and then drain the fluid with the dirt and refill with new fluid?
Not so sure I agree with this, it should be scanned on a trans function scanner and look at solenoid functions and trans codes, especially the troublesome pcs solenoid. Yes debris can cause one of the control valves to stick, which should have a history code, the vb has been somewhat problematic also in respect to the tcc valve bore.