FAO daz1989, something to try before you go changing the relay stated by markparker73. Next time it does it get something solid but not too heavy (small hammer would be ideal) and tap the solenoid on the starter motor. I used to have exactly the same problem on my Mk3 XR3i many years ago. When it used to get warm the solenoid would stick and would need a sturdy tap to free it. You tend to notice it at petrol stations because the engine doesn't have time to cool down in the 2-3 minutes that it takes to get fuel. Once mine had cooled down it was fine. If this doesn't work then try changing the relay.