Vauxhall Omega 2.5CDX TReg Dashboard Instruments faulty?

Every now and again, the Speedo and Rev Counter on my Omega "freeze" and stick in one position. The temperature and petrol gauges drop to zero. Then all of a sudden, the ignition light flashes, the dials go hay-wire and then every thing works fine.

Whats happening and where's the fault?

Thansk