Motul oil is as good as it gets, so that is not the issue. I think you just need to pull the head and get it checked out a set, head, springs and cams. I think it will be a false economy to do anything else. Someone might have cut the spring seats, the valve seats might be the problem, and you would never know. Another option is to just get a new head, get it build by a reputable shop and swap it over, that would be my plan of attach.
Graham