It's most likely a dry joint or dirty switch contact. Try cleaning the switch contacts with Servisol first.
Given that the receiver actually dies in VFO mode it's unlikely to be the display chip which is at fault, which is fortunate, as they're difficult if not impossible to get hold of.
If you haven't got a manual you can get one here:-
http://www.mods.dk/manual.php?brand=yaesu
If you need further help it would help if you could post the circuit here.
You say you're not an electronics expert, so perhaps a rig like this is not the ideal one to learn some fault finding skills on.