I have a feeling this may be a long reply, so maybe grab a 
first 
Before I go on, I should point out that although I tried the test kernel you mention in the Jerky playback and no sound on some videos thread, I reverted to the GA kernel (“…-27-osmc”, all updates applied including the ones which seem to have appeared today) and ran apt-get autoremove before any of these tests.
First things first - is it the network?
I copied “Prometheus” (3D Blu-Ray full-rate ISO rip; protection removed) onto an SD card (“Samsung Memory 64 GB Evo MicroSDXC UHS-I Grade 1 Class 10” to be exact, so with a manufacturer’s quoted read speed of 43Mb/s that ought to be fine with the ~25-30Mb/s I see if I check the info. randomly while playing the title - oh, and it’s a genuine card: Samsung lead the world in “Frustration free”-free packaging, and this took about 5 minutes to get into with bolt cutters and a blow-torch - it’s genuine …).
You can find the “MediaInfo” on the “main” title file (“BDMV/STREAM/00916.m2ts”) here, but so far as we’re probably concerned, the headline figure is “Maximum Overall bit rate: 40.1Mbps” - as it’s a genuine Samsung UHS-I card (snigger … bolt cutters …) even that should be well within tolerance.
Oh, and probably irrelevant, but I used it as-formatted from Samsung (single “exFAT” partition); no hookey “Optimise your flash!” type partitioning or formatting tools.
So what happened?
Well, started playing in seconds, not minutes; I’m currently about 10 minutes in and there has not been a single pause, skip, drop-out - video or audio. Perfect playback, and if I look at the info. I see 1 drop, 0 skips.
So, I could be wrong, but I’m fairly convinced we’re having issues here getting data in over the network …
Yes, it is the network …
I was very interested in the differences between our NFS configurations. So that’s the next rock to turn over. I’ll ignore libNFS and just stick with the kernel implementation, so I’ve got our “/home/Multimedia/” share (and “/home/dmz/”) mounted on “/home/Multimedia/”; “/etc/fstab” lines as above.
Now here’s a funny story …
I originally added the “Multimedia” share as an “nfs://…” source. Subsequently, I added “/home/Multimedia/” as a “local” source.
I can’t find any way to delete the “nfs://…” source via the GUI, and both obviously show up as being called “Multimedia”. So I tried pretty hard to make sure I was selecting the correct one.
(you can see where this is going, can’t you? …).
To make absolutely sure, I manually edited “sources.xml” and removed the “Multimedia” <source> entry from the “Video” block and replaced it with the <source> entry for “Multimedia” from the “Files” block.
Et voila …
So thank you for all your thoughts and hard work offering suggestions but it seems that ultimately @sam_nazarko was right (yes, you do have libNFS problems! ) and you, @DBMandrake, have kindly spent your time helping out a PEBKAC issue …
I am so sorry about that …