Sony VPL-HW20 Lamp Flicker?

My projector has developed an issue I haven't seen before.
During the last months while watching a movie it appears as if the lamp is flickering.
It is clearly seen in the black bars when they start flickering brightly, luckily on main image it is not noticeable (unless you're looking for it).
Needless to say it is very annoying as it happens several times during the movie.
It comes and goes away with no apparent pattern.
My question is has anyone any idea what this is?
Luckily it only lasts a few seconds each time but I think it is progressing.
Definitely not a source issue and I don't think it is the amp or the cable.

Thanks for that dragon.
A. Never run in low mode always in high.
B. My first thought as well, time to dig out the old manual, never replaced a lamp myself.
C. I estimate under 600 hours so it can't be that but I will check the lamp hours out of interest.