electronworks: It's on solderless breadboard. I know a lot of people hate the stuff but I've never had any problems at such low frequencies. Just the same, I'm going to rebuild it with good old dead-bugs and point-to-point to see if it helps.
smanchez: I'll try a snubber and I'll pile more capacitance on the TC4427. But it's just wierd to see a MOSFET switch oscillate. As far as the waveform goes, for Vdd < 5V the gate voltage looks like an ordinary 12V pulse. For Vdd>5 the oscillation grows on the trailing edge of the pulse and continues for a few milliseconds. The length of the oscillation varies randomly but the higher Vdd, the longer the oscillation. Above about Vdd>12 The oscillation never stops.
Apchar