Troubleshooting toslink input problems on DACs

I have found that the Optical connection is very non reliable at keeping a good connection between a transport and a Dac. Sounds like your are having a similar issue.I would switch to something that would allow you to use a Unbalanced or Balanced cable.I use a RCA cable to a DIP (digital processor) then XLR cable OUT to a DAC, then out using analogue RCA to pre amp. With my setup the sound is like a warm turntable very analogue sounding and easy to listen to for extended periods. I use a McIntosh cd player as a transport and I run digital RCA to a Monarchy Audio DIP then out to a MSB Nelson DAC. The signal is ROCK solid. When I was using a Sony player with Optical it was not as good.

What cables are you using?

Since using DH Labs cables I have had no problems with optical - they lock in place and are one of the fastest cables out there

My surround sound system sounded so much better once I put them In the loop.

Had them on my iMac/DAC for streaming and no problems

They sound amazing as well - they beat Chord and Van den Hul optical cables by a long way

troubleshooting ideas. as you have it narrowed down to the toslink you can swap in a new cheap cable to eliminate that as a possibility. you can swap your cable end for end and that may alter the behavior if the fiber end treatment is shot. try another toslink source like a tv of video game console and test the source. if none of those alter the problem you likely have a messed up dac.