I've noticed that my Venture MH-DE 70W/UVS/PDX pink MH lamp, dipping in brightness as if it wants to extinguish.
Ballast is Philips HID-PrimaVision PV C 70 /S CDM electronic HID ballast.
Why this is happening?

Q: "Why is it happening?"
A: "Something is breaking."

It could be a lot of things, without proper diagnostic you can never get any useful answer...
Dous it happens with some other compatible lamp too? What are the voltages/currents doing when it is acting up?
Is it sensitive to vibration of the ballast or the lamp (so broken wire or a dry joint,...)?

Two ideas:

1. (I already said this) Arc is jumping between starting coiled part and the tip of the electrode. Hopefully will go away after some natural aging.
2. Color MH often have nonstandard burning voltages. For example, from my purplish 150W lamps, one has 85V (pink) while the other has 110-115 (magenta). That may screw up electronic ballast power regulation in some way.