The usual suspects:
Run new 10- or 8-gauge fused power and ground lines to the computer and to the dash. This will stop the low-voltage alarm issue and would help if you've got a crappy connection too.
Clean out the fuse holder by the battery and any connectors you can get to on the computer and under the dash.
If you have an isolator, etc., take all that out and work as stock as you can (alternator, perko switch, battery) too see what changes.
Didn't Capt. Rick say he would test it for you if you mailed the computer to him?
Yeah, I get the annoying low-voltage alarm and my depthfinder broke a couple years ago, but beyond that, things are solid - tach, multi-function gauge, speedo and message center are all solid.
What's your charging and battery setup? Any chance it's been hard on the computer like jump starting is supposed to be?
Something's broke. If you're gonna sell it, pick a better reason, like Fest's "I need seating for 32 and twice the engine displacement."
