MC 8.4 "The System Has Run Out Of Application Memory"

I've just updated to 8.4 from 7.0.3(ish). I needed to do this to see if it would fix a subcap bug that caused randomly inconsistent drop shadows.

Now I am getting the above message regularly when I export or render (maybe other tasks, I don;t know yet).

I have opened the Mac activity monitor and it shows the memory usage creeping up continuously until it maxes out. The memory isn't being "given back" by MC so it seems. Is that a memory leak?

Anyone else?

System:

2010 12-core 2.66

24gb DDR3 1333MHz ram

OSX 10.10.3

ATI HD5870 /1024mb