What James said...
The third sentence in your original post eliminates the files as the source of the problem - if they play correctly sometimes, but not other times, then your rips are good. It's either a buffering problem in the BPD or the problem is with the disk. I'd suspect the disk drive first, so it makes sense to move your files to another drive. Being 75% full isn't the issue either. For the record, I have a couple of 1TB USB drives and one of them only has about 100MB of free space - never had the first hiccup. I suspect you've done what I've done - just ripped disk after disk to the thing (it's not like it's going to get fragmented with this kind of usage).
A 1TB USB 3.0 drive is under $100 - amazing how cheap storage is these days. When I first started servicing this stuff, a 96MB drive was $60,000!