Gilera Stalker starting problems?

We bought my son a 2nd hand Gilera Stalker but are getting frustrated by intermittent starting problems. Most times it starts first time but other times it just refuses to 'fire'. Oddly if you leave it to stand for a day or two it will again start first time. Fuel and 2-Stroke always kept topped up. We even bought a new battery in case that was the problem but to no avail. Ideas please?

Gary
May 2010