DSM300 intermittent

[Kevin]

Working on a boat with an intermittent depth problem.

DSM300 (v4.20) connected to E120W (v2.65) MFDs via a Sirius SR6 ethernet switch.

Sounder (and therefore depth on ST70 instruments) keeps cutting in and out. At various points I've gotten the following blink codes:
1 Green every 10sec
Flashing Green (when it's actually working)
4 Amber (Reserved?)
8 Amber (Watchdog Timeout Reboot) it'll show this after power cycling and the sounder is actually working. Eventually it'll drop out again and show either the Green standby blink or the 8 Amber flashes.

Went through the FAQ from 01-07-16.

Power to the sounder is steady at 12.7V, wiggled cabling to see if the sockets were loose (didn't change anything). Condition of the cabling is good. Used a new STHS cable to go from the Sounder to the SR6 switch with the same results. Unfortunately didn't have another transducer with Raymarine plug on it to test.

Transducer setting in the menu was stuck on the P66 etc (plastic transom mount transucers) I couldn't change this until I had a brief moment where the unit was operating and could change it to the B744V. (The text had worn off of the transducer sticker, but it looks the same as every other B744V I've installed).

Just want to know if there's anything I could check that I'm missing from the FAQ before sending it in for repair. Are the DSM300s still serviceable or at this point in the lifespan does it mean switching to the CP370 is the only option?

Thanks