Ford Repair: lynx/escort, mercury lynx, timing chain

Question
This one has stumped every mechanic so far. I have a 1985 Mercury Lynx (same as Ford escort. 1.9 liter, 4cyl engine.  Manual transmission. New plugs, wires, distributor and rotor. Recent rebuilt carburetor, new timing chain.
Problem:  The car bucks after it reaches a cruising speed.  This can be in first, second, third or fourth gear. No problem on the acceleration. No problem if throttle is fully released and the car de-accelerates. The bucking occurs after the car is finished accelerating and begins to cruise. The bucking is not wild like if a plug wire has burned off insulation. But it is sufficient to make the entire car shake and jolt.  The car also has a hard time adjusting its idle down. That may or may not be related.

Again, the plugs, wires, distributor, rotor, carb and timing chain are new.

Thanks for whatever help you might provide

Answer
Stuart, the first thing that I would do is replace the ignition coil.  If that didn't help, then I would change the pick-up coil.  More than likely it is not a pick-up coil, they usually just go bad, but sometimes they "limp" along.  It sounds like your ignition coil to me.  As far as the idle, you might look at your choke and make sure it is not sticking.  Depending on if it vacuum, electric or temperature based, will depend on how to adjust it.  Try the coil, and then let me know if that doesn't help.  Jason