It was indeed onion weed, Bob. When I switched to sharp blades it seemed to go away over a couple of years. The sharp ones cut it, the blunt ones didn't.
If you are mowing something like couch or kikuyu, or even fine rye grass, provided you run the mower fast you can get an OK result with blunt blades. My theory, though, is that the less you shred and tatter the cut ends of the grass, the better the job. Also, when I'm mowing easy stuff, I like to use about 2,500 rpm, and save the flat-out stuff for when the wheels really sink into the longish kikuyu in summer.
