xenon fault/surge?

[Elliott M3]

Hi Everyone. I'm new to this site however have always read the forums and found very interesting. I now own an M3 cab.(54 reg) I have a small niggle which is being looked at by a friend next week but wondered if anyone has had a similar problem. When driving along the headlights surge (go bright for about a second) then return to normal.. they work ok and the bulbs are fine. any ideas?? many thanks