If you have your receipt (or if you paid with credit card your satement for that month may be enough ) yamaha should alow you to use a local repair centre and this is what they have said on ocation in the past, although no one has put it to the test.
Send them an email detailing the problem and ask if you can get it serviced localy. I think that european law means that are covered but its best to make sure.
Have you checked all the normal problems enough ventilation, cables not touching and maybe checked for dust, maybe able to use a good hover to suck some up from the inside via the outside. I don't think opening it would be a good idea though as it will usualy void your warranty. Also check for touching wires and wires touching the case, make sure you haven't got some dodgy ground loop to by disconnecting all outside sources.