nah that's not it, compressors are supposed to cycle on and off, when their pressures get too high in the a/c system or the evaporator gets too cold (so you don't form one big block of ice on the evaporator).

It might just be normal, I drive a Jeep Cherokee at work and that compressor cycle's on and off very rapidly, but it's normal. Remember, R-134a buildsup higher pressures in the a/c system than R-12, that's why you're supposed to use no more than 80% the amount of R-12 as originially indicated to put in R-134a amount, and in that instance, I would check actually for overcharging if it's that much of a problem, you would also noticed a decrease in cooling efficiency. How do you check such a thing, sorry but professionals come into play here, as they have manifold gauges and such that compare high and low side pressures to what they are nomially supposed to be.