worcester greenstar utility18/25kw 2yo?

the above greenstar utility 18/25 has worked really up to 2 weeks ago when it cut out and the reset button had to be pressed to get it started. ever since then the same thing applies, works for 2-3 hours then stop? checked fuel flow and leaks, vented fuel for air locks, done some swearing, but maybe something needs adjusting any one with a simular problem? my unit runs on kerosine........thanks john R