Hello Fred, you need to feed the lights from the + pole of the decoder instead of the chassis.
For doing this, you need to isolate the light bulbs from ground.
Märklin has some connectors to accomplish it, those supplied with 60903 and 60904 kits.
If you decoder has a Black wire which goes to the motor, that is the + pole. If your decoder has no Black wire (e.g. 6090, 6081) you can create a + pole connecting two diodes (1N4001 will fit) from both Ground and the Pickup Shoe, with its cathodes (marked with a white or black circle) joined and pointing to the lamps, this will be your + pole.
Feed both lamps with the + pole and either the gray or yellow wire and you will have flicker-free lights.
Hope this can help.
Best regards from Buenos Aires, Argentina.

I use the 2 diodes and capacitor approach. I initially used the 1N4148 but they failed after a short time, so I changed to bigger power type diodes 1N4007 (I think)

Not sure, by no means an expert, but every website I've seen has this approach. I guess it may be some type of smoothing effect.