Power steering hard turning sometimes

Last January had head gaskets replaced and started have steering issues as soon as we got it back. It is erratic as to when it will do it. Never had the first issue with power steering before this. Most times it gives us no problem. Sometimes hard to turn left or right, or only when reaching end of turn stroke when starting into turn at slower speeds like pulling into driveway. When backing up wheel does not want to turn or turns hard either way sometimes. We were told at mechanics they broke power steering reservoir pump pulling heads and replaced it. Second pump made awful noise after one day and did not really help anything. Third pump mechanic put OEM rebuilt pump on helped but still problems. Took back again and this time flushed unit two times (said maybe dirt in system) and said thought it was good. But again started having problems half a day later. As I said does not always do it and sometimes it will do it all day and other times not at all for a day? Cold/hot engine has no bearing. Having hard time believing bad rack as it started as soon as head gaskets were replaced. Also noticed sometimes you cannot turn wheel when at complete stop. Do not know if they checked pressures or not. Any suggestions?