Two units being bad in one place sounds more to me like an issue with the location than it does with the unit. I too have RAM mounts but, like anything else, they take a beating up front. Again, I'm not sure of where yours is mounted, what type of boat you have, what water you fish and so on, but I suspect that no matter what unit you put up there you might have a similar issue.
The issue might not be while you are fishing. The issue might be while you are trailering your boat to and from the ramp. Another issue that I have seen is extreme cold weather, hot weather, or extreme changes. Living in Northern Michigan and keeping ones boat in a garage is okay... right up until you head down the road and it is 17 degrees outside. Electronics don't like that quick of a change in temperature. As a computer specialist for the government, the issue here can be simple but figuring it out can be difficult. I'll throw a couple of questions out there for you however... many minds might come up with something you did not think of.
You say this is the second unit (Lowrance). When you changed them out, did you also change out all of the wiring that went to it. I.E, did you utilize a new power cord or keep the old one in place? Did you change the in-line fuse? Is there an in-line fuse? Are all the wiring connections properly water sealed? I would say there is a common variable here and my suspicion is that it is not the unit itself. I own 7 HDS Gen 2 models in every size they make and have not had one issue with any of them. Not saying you didn't get the "two bad apples" just saying that I would think that there is another issue causing the problem.
I have OCD "Obsessive Crappie Disorder"