I know that this is an old thread, but the problem still appears, and perhaps the following will be a clue for people who care about this problem:
I have it on various versions of 6.x running on a virtualbox VM, where the host is Windows 10 and for the moment the host only has 4GB of RAM, so I only gave 1GB to the VM. The thing is: i can make the problem go away just by giving the VM 1500MB of RAM.
I believe that what this "solution" and the darling one of this thread have in common is that they are ways to deal with a machine which doesn't have (quite) enough RAM -- is that accurate?
One practical consequence is that if you do all the clean-up suggested earlier, and that works, but the problem comes back too soon, you might do better to install more memory or do something that will use less memory PERMANENTLY., not just until the cache fills up again.
I have 16GB on order, and once it's here, i will allocate 4GB to the VM. If you don't see any edit here, it's because I'm too busy having fun with my running "Process system" (sic) to post here