Sounds like youīve done the usual save maybe moving grounding points to see if it improves.
Iīve seen numerous threads with issues on the M4 and the M2 for RFI and just generally having noise issues. See this thread with Mickz entries on it. The last 3 pages have his particular solution if your set on keeping it. I think heīs got a thread on exact details on how he built it as well, but canīt find the thread right off here. General idea of using feed-through caps with an aluminium case to eliminate the noise, in his case more for the RFI that was occurring, messing with his GPS and such.
The grain of salt should of course be taken, as this model seems to be the most popular as well, so it will of course have the most posted issues. That said, I do see many having issues of one sort or another with them, some can readily be played off as user error, others not so much.
Still no guarantee really, as you might just be unlucky, and have an usually noisy alternator or noise creeping into your lines. In which case sending it back might not help, but if you didnīt note anything with the previous unit, itīs a fair bet you could get another with a bit better filtering that will likely work better in your set-up. My personal preference is an opus.
Assuming you have parts of the PC bolted, or anything else in the potential circuit grounded via the frame it can then use that for itīs ground. Which is something of course you donīt want.Another thing is that i noticed that when i took the ground cable out while the computer was working nothing stopped and continued to work with only the 12v cable connected.How is this possible?