Occasionally brakes sticking on 2002 Camray

2002 Toyota Camray, 60,000km, auto, all new brakes, rotors 5 months ago. Problem is that both front wheels are becoming locked on to the point of dropping into 2nd gear on the highway of a speed of 70kph. After a bit, this will then disappear and all returns to normal and works perfectly. Have taken apart and all is perfect no binding in pads, sliders or anything. Fluid is clean hoses good. I'm leaning to the vaccum side of things or a master cylinder where as "both" sides are locking on. No lights on the dash so I think valve is OK. Any ideas at all Please. Thanks in advance. Paul