306 headlight problem

[S34n]

I'm having a problem with my headlights on a 1997 Peugeot 306 1.6
The side lights work fine, the full beam works fine, but I can only get the dipped beam on when the ignition is on but the engine isn't running, or if I flick the switch fast past the sidelights.
But if I put the full beam on and go back to dipped beam, they go out and I'm left with only side lights! The only way to get dipped back on is to turn off the engine and start again!
Any ideas?

[Old Merc]

I think it could one of two faults I`ve had with old 306 h/lamps.First find the "dim dip" resistor,it should be on the n/s inner wing behind the battery.Its a small metal cylinder with a multi plug,they can get hot and melt the plug.The other is the collum switch,replaced many.