Toilets, Sinks, Showers, Tubs and Disposals - Weekly/biweekly toilet clog

I've moved into an old house with what appears to be a low-flush American Standard toilet in the second floor bathroom. I haven't been able to find a model number (any suggestions on where to look?)

Every one to two weeks or so the toilet clogs. It then clears easily with a plunger. No kids in the house -- just me and my wife.

The toilet is right below the roof vent so I assume that it's venter, but don't know for sure. The roof is very steep. Another toilet on the 2nd floor works fine -- also American Standard low-flush but a different model.

Any suggestions on what might be wrong? Thanks

I had a similar problem. Drove me nuts until, after a particularly aggressive plunging session, the culprit floated up. Seems my teen age daughter had been flushing her 'feminine' products down the toilet. The ensuing conversation was definately a father - daughter hallmark moment. Should have let my wife fly with that one :)