there's nothing wrong with flushing your transaxel, the issue comes in how it's done, if the shop uses the cars own transmission pump to cycle the fluid there is no problem with doing a complete transmission flush, the problem comes when shops use these high power motors with detergents to cycle fluid in the transmission, as what happens is the higher flow causes things to break off and pack up in bad places , when using the truck or the
cars pump the transmission fluid is pushed through with the same force as if you were driving it.....
as for your issue, could be the torque converter falling to drop the hammer at the right rpm's as opposed to anything being wrong in the actual transmission itself , if it was a clutch plate it would be the same thing regardless of speed and rpm's, that plate would just fail to grab/clutch most of the time through all ranges where a torque convertor can fail to get the fluid pressurized enough in certain rpm ranges and cuase slipping, i personally wouldn't worry about it these are not drag cars and if your on the pedal that much you should probably build your transmission and torque convertor to match......personally i never stomp on it, it's always a long smooth acceleration to get where i want to be, i just don't see the need to rev the motor to 5.5k , just wait for them to pass and then get over or just take a chill pill all together.....