plueders

I have a 2004 outback H6 that has 26000 miles on it. It is up to date on all of the service items. The car has a hesitation when I am going about 65 mph and up a slight grade. The car feel as if something is slipping. It does not do it all of the time. I took it to be serviced and they flashed the ECU. That seemed to reslove the issue for 2 days then it happened again. The service center has been real good about trying to resolve this problem, however they are not able to duplicate the problem. Does any one have any suggestion that I can give to the service center?