instrument lights odd behaviour

[Capt Sensible]

Hi folks - I wonder if anyone can help me with a worsening problem with my '94 986? When I turn on the headlights, the instrument lights don't come on. For a while. A few miles later and suddenly everything fires up. The lights involved are the instrument backlighting and all the switchgear illumination. My first thought was a bad connection somewhere, but the curious thing is, that once the lights come on, they stay on - I had a 5 hour drive the other evening and they stayed on the whole time. If there was a bad connection, then you'd expect intermittent operation, but it just doesn't happen that way - once the lights are on, they stay on. If you turn the headlights off however, even a quick "dip" from main beams to side lights, to thank another driver for example, then the instrument lights don't come back when you turn the headlights back on again, until a few minutes have elapsed. When this all started a few weeks back, the delay was quite short - a couple of seconds or so - but its gradually getting worse and is now about 10 minutes. I have checked the obvious suspects - waggled the lighting switch and the dimmer control, but with no obvious effect. If anyone has any ideas or suggestions I'd be very grateful for them, because this looks to me the kind of problem that is going to make my local dealer a happy man.....