The vertical storage shouldn't have any impact whatsoever, I'd have thought it'd be a completely different reason. How old is the amp?
I noted that you are in N Sields (I'm less than 10 miles away), I can put you onto a couple of very good local technicians who could give it a service (which is the least that I'd recommend if you are relying on this whilst on cruise/ferries). Better to be safe than sorry!