Irregular Monitor Flicker

A 4 year old NEC- MultiSync C500 Monitor has for some months been giving a sharp/brittle click foolowed by a severe flick that *shakes* the image up and down. This does not have a set pattern nor does it tie in with any specific activity that the PCmay be doing. The OS is W98se.
The Monitor has never had the case removed or been fiddled with and all leads appear OK with secure connections.
Thank you in advance for any clues – Errol