Can anyone shed light on my wiper problem.

On intermittent wipe all the instrument cluster warning lights illuminate plus the instruments all go to zero (oil, fuel etc) This happens about every six to ten wipes. Its as if the ignition key had been switched off. After about 4 seconds they all go out and the instruments return to normal. The car drives perfectly normally throughout this process and the problem is more annoying than anything.

I have tried changing the main wiper relay sited behind the main fuseboard but the problem persists

All suggestions most welcome
1991/2 140 V12