I've replaced front calipers, power booster.

You could either have contamination in the system or the master cylinder pushrod is not adjusted properly and is not entirely releasing. That will cause it to build up pressure.
If you want to find out if that is the case, wait until the next time they lock up and then loosen the hold-down bolts for the master cylinder and pull it away from the booster by 1/4 inch. If the brakes release, you found your problem.