Just wonderred if anyone can help as the garages round here cant seem to find out whats wrong!

Problem is every now and again the car refuses to start - starter turns teh engine ok but no spark at the ignition. If its left for an hour or so then sometimes it will start. If its given a tow start then off it goes! A jump start doesnt make any difference so I dont think its lack of charge in the battery.
The garage has disabled the alarm system, replaced the cankcase sensor and fitted a replacement ignition control unit.
And still it does it!!

HELP!! please.