Today I was heading towards a class-C, flight follow with a rebuilt transponder. I got radar contact at 3500, and they told me to fly under shelf at 1700 feet. About three minutes later, they asked my airplane type again (C-152). Turns out, my mode-C was suddenly high (and I mean on drugs). I was descending through 2200, but mode-C was reporting 6000. Needless to say, they were wondering how I got up there so fast, and how I was going to descend to 1700 before entering Charlie.