That's odd - your numbers look good to me. The cut is deeper than I'd usually go in one pass - I suggest about half that or less - I like the look of a single pass at 0.004" best, but you may need a deeper/wider line of course. I would expect it to get worse with the faster feed - was that the case? Is the tool coated (I've had trouble with TiN coated tools in 6061 Al.)? Are you using any cutting fluid?
I'm guessing more RPM isn't an option? I'd suggest trying a lighter cut, maybe squirt some kerosene or similar on it, even drop the feed a little more and see how you go. The tiny flutes on those little engravers don't give much room for the chips to clear, and without stratospheric RPM, the only way to get rid of them and not smear them about seems to be running a really minute chip load (like 0.0004" per flute). That seems to give me the best results. The deeper the cut, the more the chips seem to roll up and clog the tip. Three flutes instead of one or two won't be helping either, especially considering the limited RPM.
I've had good results using the 60° uncoated cutters from here (at 40K rpm, 32 ipm, no cutting fluid) : http://www.precisebits.com/products/...oreengrave.asp
Hope you can sort it out!