Brake Master Cylinder problems of the 2004 Volvo V70

In the summer of 2004 I thought I felt my brakes freeze up, but just thought it was me, thinking "I'm in a year old Volvo, there's no way that could have happened. " a few months later it did happen and I took it to the dealership where they replaced the master cylinder. That was 11/19/04. On 2/28/05 I returned the vehicle to the dealership again for the same problem. This time they replaced the master cylinder and booster. Then, the end of April I was getting on the highway going about 50 mph, and had to apply my brakes, and they froze up and I glided for a few seconds before they released and I was able to push down on them (this is what happened the other times. ). Volvo again replaced the mater cylinder and booster (and they told me they replaced another part that had to do with the pumping mechanism (?) "just in case. " the car made it 4 1/2 months before it happened again. I was getting onto a highway 9/9/05, going about 60-70 mph, when the car in front of me applied his brakes. I applied mine, and for a split second they froze. I accelerated again almost immediately and then had to slow again. This time the brakes froze for about 3-4 seconds before they released and I was able to slow down. It is in the shop now and Volvo assures me that this hasn't happened in a new car before. My question can they recall something they can't even fix???.