Super,
A slow oxygen sensor could be your problem. Since the problem is worse coming from high speed, the problem could be your O2 sensor being too slow to react to the oxygen levels in your exhaust at idle right after cruise rpm. A yo yo effect then happens as the mixture and oxygen levels chase one another until they stabilize. I had a similar problem, my O2 sensor was fried from burning gallons of oil through bad valve guide seals.

Your problem could be many things, but it sounds like you have just done a tune up and if you have more than 60k on your Oxygen sensor (or don't know when it was last changed) it is probably time to replace it just like the wires, cap and rotor. Once you eliminate the obvious things then you can buckle down and start looking for leaks.

My 2 cents.

I attached a link with some O2 sensor info.

Good luck,

Bob

http://www.***************/html/emissions-oxygen_sensors.html