Mystery Horn Problem

[Betazag]

Hi. I have an 82 Beta Zagato Spider that I have been restoring for about a year now. I have a problem that I need help with. My horn will blow fine when the car is not running but while I am driving, it quits working. I have taken the steering wheel off and cleaned all of the contacts; I installed a new cluster(lights, blinker, wiper controls as well as horn contact); I installed a new horn compressor; I cleaned the fuse contacts and put in a new fuse. The relay seems ok but I am thinking about trying a new one. I cannot figure out this problem and was wondering if someone might have a suggestion. Any help would be appreciated. Thanks, Mark

[Hamish]

Have you run a circuit tester over the horn wiring with the engine running :?: If not get a volunteer try the horns then check the power feed..... sounds like it could be earthing, but thats pure speculation :(
Any other systems not 'behaving' :?: