Start by checking crucial things like oil level and coolant level, both are important to cool the engine down. Next step is to look at the spark plugs, if they've been in for more than two years change them. HT leads can be a problem especially if they get wet. Spray with WD40 or similar to get rid of moisture. If you lift the bonnet when it is dark with the engine running you can sometimes see if an HT lead is shorting out instead of going through the plug. Good luck.
Richard
December 2008