There is two possibilities here that I can think of.
The first is that your arrows are underspined. To confirm this try dropping the draw weight to 60# and see if the bow will tune any better. If it does then you pretty much have your answer. Either shoot less draw weight or get stiffer arrows.
The second is that there may be some lean in the idler wheel. If so then you need to correct this. There is a forum on Mathew's website, I believe, and several threads on just what to do to correct it. I would do this first as it should be part ofan initial setup to begin with.
And I agree with your assessment of your present dealer. I think I'd be looking for another one; not necessarily a Mathews dealer, but one who understands that an arrow should leave the bow in a straight line and stay that way.
Hope you get this worked out.