We've been experiencing similar sporadic and unexpected QVS service failures. They are usually occur when the RAM has reached the QVS lower working limit.
We have the Service set to restart on failure under the Recovery tab, but it is still annoying to customers that lose their session. Especially, if it occur repeatedly over a short time frame.
We have been working with Qlik for months, but still don't have a solution yet.
They indicated that applications with Direct Discovery could be causing the issue. Are you using that in any of your applications?