In response to Hobbipro...
1) Disagree - you should get plenty of life out of a four-cell pack - certainly more than a couple of cuts - it's only spinning a motor with a very light load (assuming you're making shallow cuts and not plunging the cutting bit deep into the comm) - it may just be that the pack is past it's best.
2) Disagree - may as well take the plunge with the diamond bit straight away if you can afford it - not only does it last longet, it should give a better cut as well
3) Disagree - serrations are only there to help bed a brush in - you can run a worn brush on a freshly cut comm, just give it plenty of time to break in (running in on the 4 cells) - but brushes don't cost much, so you may as well make the most of the power boost a cut gives.
4) I don't know what a sharpie is
5) Disagree - if you can spend a minute doing a single cut, you must have the slowest and steadiest hand known to man - just a gentle constant motion across will get a good shiny cut - taking about 10secs say - and I cut both ways as well.