... as an update ...

This weekend I had a bit more time on my hands, so I lifted the air cleaner to get proper access to the cables, linkages and vacuum pipes. I noticed that the accelerator cable wasn't quite returning to its proper rest position and therefore not closing the microswitch (not sure what the function of the switch is). I lubricated all linkages and now it's closing the switch properly.

Apart from that, I can see no other problems. I inspected the vacuum pipes and they seem to be OK. The rubber ends are definately not brittle. So.... my original problem still remains! Help please!