Serviio

I've started having a problem whereby my movies stall when running Serviio on two LG TVs. My set up is:
Serviio running on Mac OS Sierra (2009)
3.06 GHz Intel Core 2 Duo
RAM 12 GB 1067 MHz DDR3
NVIDIA GeForce 9400 256 MB

Movies/TV stored on USB connected hard drive
The LG tvs have the LG TV/Player profile selected in Serviio

I have an LG TV downstairs wired connection and an LG upstairs connected via Wi-Fi. Lately I've had both upstairs and downstairs stalling with loading error and stalled movie. It hangs, then plays a little, then hangs, play a little and is very jump and then hangs. I stop and restart and it does the same thing. I've been running Serviio for several years without any real problems but this is serious and making movie nights unwatchable. It mostly seems to happen when I'm running two streaming simultaneously. I have a kids movie running downstairs and our adult movie running upstairs on the weekend so it's a big problem.

I'm not sure where to look for a solution. Do I check my router settings? Or do I disable transcoding as maybe the Mac is a bit old and having trouble keeping up with the dual transcoding? I could use Handbrake to code everything as MP4 for direct playback without transcoding.

I don't think it's a wireless issue or a wired network issue as iView works fine on the TVs.

I've read through the forum and can't find this problem elsewhere?

If you could give me some problems on where to start trouble shooting that would be so helpful.

Are the files being transcoded? The native LG profile doesn't transcode unless you turn on burn-in subtitles.
What is your CPU on the MAC doing when this happens(CPU utilisation)
Is your MAC doing anything else when streaming (background applications).
What changed?

The activity monitor shows Java is using most CPU from Root. It seems to vary between 5 and 40%. I've disabled subtitles and transcoding. Nothing else is running. I'm now worried my USB drive is flaky and that's the problem. I shall move some files to my HD and NAS and check again. Thanks for your reply.