When I switch the inverter on, nothing happens for a while, then I can here the faint clicking of a relay. It seems to be attempting to switch on and I can see 230v appliance - a computer monitor - switch on briefly as it attempts with each click. Eventually it switches on. After that I can switch it on and off completely normally. It's as if it needs warming up. This has only happened recently. Batteries fully charged etc etc. Installed on a narrowboat. Any ideas?