SOX lamp flickering

[Beta 5]

Anyone else had this?
I've been converting a bulkhead to run 35W SOX in the last few days and used a Philips BSX 355 L82 35/55W magnetic ballast paired with a Parmar PB035K245 ignitor.

While testing it I've found some lamps seem to flicker quite strongly once fully warmed up, it is quite noticeable and can be seen in the are illuminated by the lamp as well as when looking directly at it.
Interestingly when the ignitor is taken out the circuit while live, the flickering reduced down to what you'd expect for a lamp driven on 50Hz mains. There is also a noticeable change in the sound of the ballast when the ignitor is removed from the circuit.
I have also tried various Thorn and Philips ignitors and other lamps with varying results, some seem to work ok while others still flicker.

Will the flicker just go away on its own if run for long enough or is there more at play here? I'm sure it is not rectifying, there is no "angry" buzz from the ballast and the flicker is not like that of a rectifying fluorescent lamp (and in fact is almost akin to the shimmering of a cold T8 lamp on mag gear), and the lamps are not EOL or close to it.