Upward curling corners / sides? (no replies)

Upward curling cornerns / sides?

I'm printing with a Replicator 1 Dual, and so far prints are pretty good, but I have an issue which I cannot explain.
When I observe the printing process, I see that the corners (eg. in my example the 50mm tower calibration tower) curn upwards, as if maybe the nozzle is pulling it up or maybe something else is causing it, I don't know.

See the pictures for illustration. There are pictures of the 50mm tower during printing, the half finished print, the precision block and also showing a tower model where you see that the corners curl up somehow. This is not the Z layer change artifact as it is visible not only on one corner.

I have tried playing around with the following, to no avail (same issue of corner curling):
1. Decreased the temperature from 225 to 215
2. Decreased the HBP temperature from 110 to 90
3. Print at lower speeds (original feedrate was 80, using Sailfish), at 40mm/sec
4. Decreased the maximum Z speed to 16mm/sec (not sure if Print O Matic overrides this)

Skeinforge using Print O Matic with some overrides:
- 10%
- 0.27mm
- 1 shells

I'm using a yellow 1.75mm ABS filament.

Any ideas? :)

Thanks!
Bence