The relay is either the fuel pump relay or the main engine management relay. If the engine stops suddenly then it is probably the engine management relay. If the engine power dies away gradually then it is the fuel pump relay. Given the age of the car I would guess that the terminals of the relay are corroded which causes excessive heat build up due to high resistance. After 500 metres the heat becomes excessive and the relay begins to switch off and on hence the clicking you describe. Once the relay has cooled down the engine will start and run for a further 500 metres until it fails once again. Remove each relay one at a time and clean the terminals gently with sandpaper before replacing.
Dann
August 2007
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