I've shot hundreds, if not thousands, of images using T400CN, and I've never had a problem like you describe. I once had a problem with the early Ilford version, XP2, where it had pinholes in the emulsion, which created very distinct, sharp dots. I'd try going completely glassless with your negative carrier, and I'd also suggest (assuming you are using a condensor enlarger) disassembling and cleaning the condensor assembly and mixing chamber, just in case you're getting some dust floating around. What would be most helpful in trying to diagnose this is if you could post some examples of the problem - can you scan in a print or two?