Brakes apply themselves

After driving about 100 miles or so my brakes start applying themselves which makes the vehicle run rough doing about 70 miles per hour, the way I get this to stop is I pump my brakes a couple of times and this corrects the problem for awhile. Then after the incident and I get back up to speed within 20 or so miles it happens again, this goes on and on. I have taken the vehicle to the mechanic and after running a diagnostic they can not seem to find the problem. When I just ride around town I never have this problem it only occurs on long driving trips over 100 mile or so at a higher rate of speed. Thank you for any help you can give.