Weird issue I’m trying to nail down, printing at 100mm/s with a 10% slow down for layers under 15secs, this happens, you can see where it slows down near the top layers. Printing in NGEN, at 220°C, which is the bottom end of the recommended printing temps, it seems that it’s not being cooled enough, am I right in thinking this?
Printing slower means you have more chance of heat creeping up the filament into the cold zone, so you might want to make sure that you are not grinding filament or anything. I have been having alot of issues with that. I do not know if that is a problem for you or not.
PLA is pretty much the only filament that needs active cooling (fan). You may want to try without cooling.
I have never even heard of NGEN before.
@NathanielStenzel - @colorFabb_Filament 's NGEN. The Extruder is tight, due to a short bolt, I have some 40mm Machine screws that have come in yesterday to my work, so it’ll be swappped out for those.
@Carlton_Dodd - NGEN needs Cooling, as recommended by Colorfabb. It just gets worse as the cooling speed is lowered.
I have noticed this effect when switching speeds during a print. I don’t think it is a mechanical or cooling issue though.
Most slicers set feed and flow rates equal and I know that material behaves differently with respect to speed and temperature. I believe this is what you are seeing i.e. you slow down and the material behaves differently for the same temperature.
Try printing the part without slowdown but with cooling. I suspect that, at a consistent speed, you will get a better print.
@Neil_Darlow - that was a thought too. I started to try it last night, but went to pull the plug on the Migbot I have here and pulled the plug to the ZBox computer running Octoprint… many expletives were uttered. I have something else running at the moment with less slowdown, but I’ll give it a shot.
You are either printing generally too cold and/or the difference in printing speed is to big.
It has to do with calibrating your flow with a given speed and temperature. Now you changed these parameters too much and the flow is reacting to this change. Printing slower means higher temp for the feeded filament. If you calibrate your flow with a higher temperature, these effects get smaller.
Mathematically, Filamentflow is depending on temperature like a graph of f(temp) = 1/(temp - 1)
@Rene_Jurack - thanks, I had a feeling you’d pop up here 
It’s better since I disabled cooling slow down and upped the temp by 10c but still not quite there, I need to have a dig into the settings a bit more and work out what’s going awry… As for the formula, my maths is pretty much non-existent and you’ll need to give me an example for me to work my head around it.
Last print was better but there was blobbing on the corners where the part fits into the metal enclosure. I think I may have tracked down the culprit though, the outline underspeed was set to 50%, I’ve raised it to 90%… we shall see what happens.
If you can figure out the layer, where the slow down begins, then in S3D you can set a lower temp (a decrease like 5 to 10 deg) at this layer and up. This should decrease the affects of overextrusion.
English is not my native language and I would stumble upon explaining math in english 
Funnily enough, I’ve pretty much cracked it, reducing all the slow-down settings has cured part of it and releasing tension on the extruder by a substantial amount, it now has the correct bolt length, has helped too. It also got a lot better as soon as I put an Aluminium plate on the bed and put the glass on top of that, although it could be a coincidence as it was done all at once.