Halogen spot lights don't always work?

Hi hopefully someone can help me with this issue! My kitchen lights when switched on will sometimes work no problem but other times you switch on and nothing happens you can hear the buzz of electric running to the fitting and when switch off it stops! It can be a case of switching the light on and leaving it then anything between 10 minutes to 8 hrs later it will just ping on!! Any ideas??

Martin
January 2017