your best course of action is to talk with the service manager and find out what options he will offer you, once you explain this dilemma. Sometimes, when a field tech is not able to find his way, the service manager can visit and set him straight.
If this is not an option, you may be better served by getting a recommendation for a company from a trusted friend and have THEM come out and have a look. Tell them what you know, and ask what it will take to find out the cause of the problem.
Then, proceed form there.