What are the specs for the servers?
First thing to look at is what the "freeze" looks like: do get crash reports in the relevant OS logs?
If you don't have the FMS stats logging turned on then do that now so that it can collect good info on how FMS performs against the 4 typical bottlenecks (processing power, memory usage, disk i/o and network throughput).
Any DMP files in the FMS logs folder?
You need to work with the real stats.log, not the stats viewer in the admin console. The real stats.log will give you alot more detail and the ability to search etc.
That machine has a single processor with 4 cores (plus HT); that's extremely marginal for 50 users and it shows in the stats that you are showing us. The elapsed time per call is maxed out around 30,000,000 which is how high as it can go given the default logging interval of 30 seconds.
Do check to see if there are DMP files in the logs folder.
To solve this issue:
1) I do think you need a beefier FMS box, more cores and that may alleviate the problem and increase the interval between these freezes
2) find out what the users are doing in your solution at the time of these freezes. There are very likely some design changes can make to reduce the load. A typical scenario here is a number of users doing searches on an unstored calculation at the same time.
I do assume that when you reboot the machine, you go back to a backup? Especially if the files come up as improperly closed / needed a consistency check?
We have a similar problem but much lower usage on FM14. We can create a 2 minute freeze situation with only 2 FMPro14 users on our server which has single processor with 4 cores. However doing the same actions using FMPro 12 against the same FM14 server doesn't create the situation. This problem didn't occur at all with up to 7 users on FM Pro12 against FM Server 12. The server processor is not showing any stress at all during these times of freezing - typically running at between 1 and 2%. I have been working with FM support to try to find the cause of this problem but no joy so far.