Also, may be for experiment sake, if available, try different brand of paper and freshly open pack. Looks like its ghosting but because only one line is affected - probably not the case. May be static. There are some limits to how much environmental and media humidity/dryness printer can handle, extreme values to either side could affect print quality in very odd ways. Also, looks like there is quite a bit of toning, unless paper brightness is so-so.
Once I had customer called me few weeks after I sold him envelope printing system, saying that he prints B/W #6 envelopes and every 5th-6th-7th envelope there would be inconsistent image density in one line, similar to what is pictured. May be not exactly like here but something similar. I was going nuts with that issue, I brought another belt, set of drums, fuser, and it would be still doing that flaw... You know what solved it? I went to near-by Staples store, bought a package of every different type/brand of #6 envelopes and guess what? All those different brands of envelopes, ran fine, without any flaws.
There was only one thing to do, is advice customer to contact his supplier and exchange those envelopes for different brand, hi did and issue never came back.
