I think there is something wrong with my suspension but my usual mechanics have been unable to solve it. At higher speeds, the vehicle can begin rocking back and forth and, if I take no action, can can begin to fishtail. I can make it stop by firmly braking briefly. If I don’t brake and keep the same speed, it gets worse. It doesn’t do this continuously but it can be started by a slight change in the pavement, a bump, etc. This can happen in any weather and at slower speeds too, but it happens more noticeably at higher speeds. Shocks, struts, and stabilizer bars have been replaced with no change. It seems to be slowly getting worse over time.
You don’t mention the tires. Are they in good shape and properly inflated? How is the wheel alignment?
There is a known issue of broken stabilizer bar links on older Pathfinders. These links are short components that attach to the actual stabilizer bar at a 90 degree angle.
You did not tell us that age of this vehicle, but if it is one of the “older” Pathfinders, then you have perfectly described the symptoms of broken stabilizer links on these vehicles. If the stabilizer bars were replaced, but your mechanic did not replace the stabilizer bar links, then he ignored the actual source of the problem, IMHO.
Based on the symptoms, I think that the problem is with the stabilizer bar links on the rear suspension, but I would suggest checking the ones on the front suspension as well. And, you might want to find a new mechanic if your current mechanic managed to overlook broken links.
Because this problem will only get worse over time, and because it is a definite safety issue, I would suggest that you get it attended to right away.
What year Pathfinder??