I have the same problem when printing the Calibration tube from a Netfabb profile (high quality) Strong/Medium.
After about 6 layers it starts to go bad and gets worse each layer as hot plastic goes on top of layers that are getting hotter and made worse with the buffer overrun and the machine pausing.
Mostly solved it by changing the B setting in the M205 command of Marlin from 20,000 to 40,000 or something like that (not with my machine at the moment to check) this setting deals with the buffer overrun but effectively slows down the printing time of a small layer.
Here is a good Post about changing stuff in Marlin https://groups.google.com/d/msg/ultimaker/jzFDsHy0XVc/MzVXUxBNlcwJ
If you are using Marlin then this might help else you need a way to slow down how quickly it prints small layers.
Printing with less heat works a bit if you can still print OK else some more cooling will be required as you say.
Alternatively print the whole model slower or slow down the rate with Repg etc. manually when it gets to those layers should work also.
Please ask if you need more help with those Marlin settings etc.
Good luck.
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IRobertI 520
I'm using the same duct as you and I have the same problem. Last time I printed something tiny I did something a bit silly... I got out a trusty drinking straw and blew as hard as I could I looked like an idiot of course but it sure did help quite a bit. I've been thinking about the possibility of adding something like an aquarium air pump and having a small hose clamped onto the print head like the bowden and then direct the air flow just under the nozzle.
But I have no idea what kind of air flow a pump like that can produce.
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