When offering support, would it be possible for you to tell us what the problem was, rather than just telling us it was fixed? That way, if it was something we did we can possibly resolve it ourselves, and it would leave us slightly less frustrated.

We tell our customers what was the reason of some issue if it was at our side.
Also outsotred support is little bit another thing - it make your customers believe that our support team is support team of your own company.