Digital BW, The Print

I'm using an R1800 with the 3MK method. The 1st black has a small imperfection in the 
nozzle check. The stair-stepped lines aren't even like they should be. Some are the little 
higher and some a little lower. 

When I switched it to a different slot, the nozzle check pattern was perfect, so it looks like it's 
that one head. Am I looking at a clog? It appears to be affecting print performance.

Any advice is appreciated.