What is happening here ?

[American 3D Printing]

Wpilgrim is most likely correct. Since Rep 2s already have an awesome fan that blows like crazy right onto the build, it's just a matter of making some prime pillars in the same build. We make smaller, taller objects all the time at the standard Rep 2 Temp of 230°C. You just need to throw a couple prime pillers on there.

For those not familiar, a prime pillar is a sacrificial object, normally a cylinder a couple cm in diameter that is the same height as the object to be built. KISSlicer can make them automatically, but for other slicing software (like Makerware for example) just make one with your favorite CAD program and drop it into the build. Some builds require multiple prime pillars.

What the prime pillar does is make the nozzle go somewhere else on each layer, allowing time for the last layer of the build itself to cool and harden before the nozzle returns for the next layer.

There is another possibility too though. The way the Rep 2 wiring is designed, the X axis wires get flexed a LOT from Y movements, and they do wear out and break eventually. You'll start getting intermittent X skips when this starts to happen. Makerbot redesigned that section of harness partway through the production of Rep 2s so that just that section of harness can be replaced, rather than the entire X,Y and Z harness, which is a true PITA procedure.

[Mouser]

Well, I printed 3 at the same time, as suggested, and they came out fine !

now that you guys mentioned it, it does seem like a cooling problem. Maybe I need to check the operation of my cooling fan, maybe it was only working intermittently ??

thanks for taking the time to respond