Thanks André - I'll have a check. The battery is good but suspect a poor connection is probably the case.There could be several causes. Is the battery in good condition? Perhaps take a look at all the battery connections, including the earth ones, particularly the one earthing the engine, and confirm they're all tight. are the connections on the starter motor tight, as well? The solenoid feed from the ignition key needs to be tight, too.
Have a rummage at these things first. It's often the obvious things that cause the problem!