Sounds to me mate, that you are providing a great service and have probably done so for some time and these people have grown used to what they consider a pretty cushy relationship, with you doing all the hard yards! I suspect apathy has set in and they've forgotten just how good they've got it, what with you doing a lot of the leg work for them.
I say, make them do the work, if they really want your skills, and sounds to me like they'd me mad if they didn't, then they'll come to you, they'll transport parts, they'll pick up parts, let them do all the stuffing around, especially if your only likely to see them every 7 months or so. It just aint worth the stress.
If your relationship with them does go pear shaped, just spread the word around why and what you've had to put up with so that other customers can see what's been going on and will stick with you, give it some time and the original problem will come crawling back. Sometimes, you gotta let these dickheads fall down so they can see just how high they've been flying!
