It's definetly a clutch problem. A friend of my bro's was complaining of a sh*tty clutch and brought it into my Auto class for some diagnosis. Drove it around the block a few times and the clutch wouldn't engage <4500. Same problem you had. In fact, after the class was over, on my way towards the school parking lot, the clutch completly blew. White smoke = not good.

Apparantly he had been driving with the clutch like that for a few weeks. I replaced is clutch with a basic, new clutch kit from Napa. He also had the flywheel resurfaced. After that, thing drove like a charm...
BTW, it was on a '89 240sx base with around 170,000 or so miles.