From my Raspberry Pi:
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I often have between 70-95%cpu on java, sometimes just 0-3%.
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i start with -Xms320m -Xmx450m
memory usage at the moment is 71.3
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got a few "generated invalid block"'s (maybe 200-300?) with followed "...Done"
That's a little too many invalid blocks. Do you have trouble syncing up the blockchain? Are you hitting a lot of forks?
The full load for several seconds is normal behavior on the RPi. The machine sits basically idle at 1-4% when nothing happens, but gets fully loaded if something happens (new blocks, orphans, you unlock your account, etc)