I have a Belgian 50W Philips CDM lamp (ED shape, medium base) that has a cylindrical arc tube. It works really well and gives a good quality light (842 or 942 colour). However, the base has a split on one side, it extends from the lamp end half way into the threads. Also, there are blueish flashes in the jacket when the ballast attempts a hot restrike. Should I retire it (it is NOS and just used for maybe 50is hours, if that) ? I can't take a picture at the moment as it is lit in my MH table lamp.

The cracked cap is not big issue, i have few SON lamps with same crack that have been going strong like that for years. Dont overdo screwing it into sckets

The breakdowns in the outer i dont think they should happen - Maybe the arc tube is slowly leaking (or the outer is, but that would be way harder to strike). If you can get ignitor with lower output (like superimposed ignitor rated for SON only) this may solve the problem

The cap I'm not too worried, however the internal arcing is more worrisome. I don't use this lamp very often, mostly when I install my MH table lamp on the kitchen table for a rather long paperwork session. Otherwise, I remove it from the table lamp and put in a 3000K one for general use.

If it becomes hard to start or if it doesn't warm up fully, then I'll know it has started leaking. Might I also notice a blackening in the jacket if the arc tube is leaking ? I would also imagine the getter would change colour if the outer jacket itself is in fact leaking.