[tor-relays] Yet another underpowered relay?

[Logforme]

I recently upgraded from an adsl line to a 100mbit fiber connection.
Naturally I want to use most of this new bandwidth for my non-exit tor 
relay. However, I run into messages like these:
Jul 22 17:40:26.000 [warn] Your computer is too slow to handle this many 
circuit creation requests! Please consider using the 
MaxAdvertisedBandwidth config option or choosing a more restricted exit 
policy. [665 similar message(s) suppressed in last 60 seconds]
Jul 22 17:40:26.000 [warn] Failed to hand off onionskin. Closing. [1509 
similar message(s) suppressed in last 21600 seconds]

Relay fingerprint: BE71 DC9D A443 5855 FAE1 E369 7080 1A60 0D51 3684

Relevant torrc settings:
BandwidthRate 10 MB
BandwidthBurst 10 MB
NumCPUs 2

I did some research and came up with the following things to try:
Set "MaxAdvertisedBandwidth x MB"
Set "MaxOnionsPending 250"
Get a faster computer / Get a computer with AES-NI

I tried setting "MaxAdvertisedBandwidth 9 MB" for a few days which 
didn't seem to do much difference. This is only a 1MB reduction from the 
10MB i use as a hard limit. Too little? Must I wait longer for the 
change to get spread around?
Setting "MaxOnionsPending 250" seem to have reduced the number of log 
messages, but not eliminated them.
The computer is somewhat dated but not ancient: Intel(R) Core(TM) i7 CPU 
930 @ 2.80GHz, 4 cores with HT, 6GB mem. I feel it should be able to 
handle a 80 mbit bandwidth but have no hard data to support this.

Looking at htop tor uses 2 cores. One core is constantly at around 90% 
(when all the 80 mbit bandwidth is used). The second core is hardly used.
Since the relay can handle 80mbit (and higher) without messages I assume 
the problems only occur when the number of circuits get very high.

Questions:
What value should I set for MaxAdvertisedBandwidth?
What other torrc settings could improve the situation?
Am I wrong in thinking my cpu should be able to handle 80 mbit?
Would upgrading to 0.2.4.15-rc-1 help?

Weird sidenote: If I start iftop the first core drops to around 40-50%. 
Stop iftop and it jumps back to 90%. No change in traffic. What could 
that mean?