Switch the machine off at the mains.
Take off the lower (metal) part of the front cover by undoing the screws which hold it in place. You will see a black rubber pipe about a foot long and running diagonally across the front of the now exposed innards. Remove it and blow through it. It will contain gunk (usually oil/fat which has congealed at the low temps). Wash the pipe through with warm water, and then reattach it, replace the front cover and all should be well. Cost me £90 to find that out. Works like a treat every time. Clearing the fan vent may help too, as described above.
stevieboy, August 2010