It sounds to me like you have a dealer problem, not a machine problem. The 870 is a top of the line machine and should work flawlessly. If you are bringing it in that much your dealer should be fixing the machine or teaching you how to use it, not blaming you.
There is a Yahoo group for Viking Sapphire owners. They are very good at troubleshooting problems and helping quilters get the most out of their Sapphires. If you search for "Viking Sapphire" in Yahoo groups you'll find them.
I have an 850, and I ended up so frustrated with my dealer that I bought a Bernina from a different dealer.
Good luck to you!
Janet