I bought one of those generic grounding cable add on kits from ebay for like $20. They have like 5 long ground wires and I just ran those things all over my engine compartment just to rule out and grounding issues. I personally recommend it as worth trying.
My rover also had an issue with a loose connection underneath the transmission. Wouldn't hurt to get a visual on that area.

 

Replaced the battery primary ground cable with another used (but slightly better looking) one a few weeks ago and so far,.....have not had the flashing M/S return. I did also wire brush all connection points again in the process.