[jostafew]

I just had a similar issue; was temporarily running a 250w 20k mogul lamp (not sure what it was, think it was a Radion) in a temporary pendant, powered by a Vertex electronic ballast. When I went back to the permanent fixture on the same ballast running a (new) cheap eBay DE lamp it would start fine but after warming up would begin to flicker a little. I gave it a good couple weeks to see if it would break in, but always the same behaviour. So I swapped in an old Phoenix lamp on the same fixture and ballast and it did the same thing but not as badly. I then went back to my PFO HQI ballast on the same fixture and now both of those bulbs run fine, so in my case it was the ballast acting up.