I think the problem is very likely that there is simply not enough utility RAM for the kernel to keep track of the filesystem data structures for two very large filesystems as you have there.

Increasing the ReserveCache= setting should help a lot -- every count of 16 represents a megabyte, but then that memory is taken away from the player's own cache (and thus the root of the problem: player s/w trying to maintain a duplicate of the kernel's own cache, thus taking near 2X the RAM in doing so).

So, try perhaps ReserveCache=48 -- I don't know how much RAM it really needs, but that ought to quiet things down a fair bit. If it does, then installing the RAM upgrade kit will help a lot as a longer term solution.

Cheers