2003 XK8 convertible electrical problem

54K miles, 4.2 v8 engine:


AT 60-70MPH, the six round dials on the dashboard (Tach, Speedometer, fuel, clock, battery, oil pressure) will dim to the lowest level (as if I dimmed it with the dimming switch) without going completely black. No other lights go dim. Then in 5-10 minutes, they can come back on fully lit.

The dealer says there is no repair code and can't figure out what is wrong.

Any ideas?