Just an fyi for those who might be interested...
With upgrading to 0.3.5.8 the memory usage of my Tor relay dropped again to reasonable levels. It suddenly started to increase end of January causing my relay to crash twice before leveling around 45% (a week before I had an ISP outage). Now it's back to around 25%. The relay bandwidth consumed over all the time was pretty much stable.
[Memory Used.png]