I figured I would let everyone know, my 99 SVT with 230K miles developed this problem. At about 1,500-2,000 RPM's the car would buck and until it got over about 2,000 RPM's. If I eased through the RPM's the problem wasn't noticeable, but as soon as I was under moderate to heavy engine load, the bucking would start. Plugs were replaced about 10,000 miles ago with OEM units. Wires were replaced 130K miles ago with OEM units. The problem probably started a few hundred miles ago. I went and just replaced both plugs and wires this weekend. Problem immediately cleared up and so far has been gone for 2 days.
If I had to guess, the connection between the wires and coil pack is where my problem was coming from. The two clips on each wire don't want to stay seated into the coil pack. I replaced the valve cover gaskets a few months ago and had to take off the coil pack. My guess is I knocked one loose during that time.
