While his interest in your service working as it should is important to him it is just as important that his stop by your house doesn't cost him more than he made while he was there. Your job has his name on it now. If problem reoccurs within 7 days and he doesn't have a chance to fix it personally it will cost him big. Days 8-90 it will still cost him, but less. Trust me......the $$$$ are his real motivation for getting it fixed. Sounds like he threw just about everything he had against it, except a new line to that spot. An ohmeter used to test impedence of that stretch of cable might reveal the cause.