I suspect the fix is simpler than you think. The car is fine for a while after being in the shop. It pulls after being driven for a while. Vibration and pulling are most probably from a caliper that is not releasing fully on the front right. The fact that the car is fine for a while indicates that the caliper itself is likely not to blame. I would try the simplest fix first. Try bleeding the brakes with fresh fluid. The age of the car and the fact that most people never change their fluid until they have a brake component replaced suggests either a small amount of air in the line that is expanding as the system warms (least likely) or alternatively a small amount of water that is boiling causing a small vapour lock in the line (my money is on this one). In any case, try this first, as it is dirt cheap. Let me know how you make out.
Those who suggested a warped rotor could be right as a result of the dragging on that corner. That is easy to check when you have that wheel off for bleeding.