I had this problem last year then again this year when it became permanent. It seemed to happen frequently on very hot days. Well after waisting money like everyone else having wiring checked and the ignition barrel replaced, I decided to send my Instrument Cluster/Panel (which is very easy to remove yourself) off for testing .
Turns out the main motherboard PCB had moisture damage and a refurbishment was required. They did have the same model in stock for £137, but since I had sent mine off already and would have to pay for the diagnostic anyway I decided to get it repaired which cost me £200.
I have not received it back yet, should be in the next week, but will post here when I do.
Heres hoping this sloves the problem.
For anyone wondering, I sent it off to the following people:
http://autotekelectronics.com/autotek/
Updated 06/09/12:
Well I got the instrument panel back and have been using the car for a week now with no problems at all, looks like it is fixed for now.