Thats a bit odd
Taking a wild stab in the dark it sounds like a scheduled task is running at a specific time and it is doing some odd with user stats.
Have a look around what task you have running when it spikes, it might also be worth putting the no plugins setting into config.php when it is spiking to see if it kills the spike. If it is then the problem is with a mod.
What sort of caching are you using?
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