You're putting too big a load on the ramp and upsetting the oscillator. The resistor should be
huge. Alternately, since you're using B-E junctions for your voltage drop, just power the collector so you get an emitter follower.
AcHmed99, I didn't even think of that circuit.
Thanks.
By the way, the reason I suggested 3843 is that you want to run pulses of current through a coil, and that is its purpose. Normally, the current sense pin's input is exactly that. You put a small resistor between the FET source and ground, and connect that to the pin, so that it sees a ramp proportional to the coil current. When you've got your coil stuff up and running, try that. You can, of course, generate pulses with a MCU or a 555 (for high frequencies, use 7555), but depending on what you're doing with the coil, a PWM controller with an analog feedback loop could be very useful, and it still does decent PWM without that.