There may well be a "cold fault" - an electronic component that will not work correctly until it gets to a warmer temperature - might also be a poor joint but I would doubt it given what you say.
Ideally it will need proper testing - fault finding with a circuit diagram and test and measuring equipment - however - thare are techniques that may find the problem without circuit following...
With the covers off and the unit working correctly after it's had a chance to warm up - freezer spray may be squirted on selective parts in an attempt to bring the fault on - freezer spray may be had from hobby electronics stores for a few £ a tin...
http://www.maplin.co.uk/images/Full/re77j_new07.jpg
Good luck...
Peccavi, July 2010
