If there's anyone with an idling thecus n2100 with 512MB ram, please can you try building /usr/ports/x11/qt3 and let me know whether it completes successfully? (<=256MB works perfectly ok whenever I've tried - qt3 isn't the only one with problems, but it fails more often than not and since it's near the start of a SUBDIRLIST=plist/arm build I have more experience with it).
I am seeing (on mine and another machine I have remote access to) - internal compiler errors, bus errors, sometimes loops at 100%cpu in gcc, libtool producing files without headers it then refuses to touch again, various .core scattered around, occasional panics. Restarting builds sometimes clear things, sometimes not. It seems to work slightly more often if PIPE is unset. I'm wondering it is indeed RAM-dependent (I think one machine reliably building qt3 multiple times would be enough proof of this I think) or if it might be some other problem common to all 512mb n2100, since I've tried a number of good sticks of RAM (various types, including some with the same speed and cas latency as the OEM 128mb and a working 256mb).
