I have the same prob with my Giulia GTV. I just got my car back on Monday after having the head done and the idle was unbelievably smooth. However, after driving for about 10 minutes on the highway, the idle roughened and the tach is doing this slight bouncy thing on its way down from revs.
I've just spent a substantial amount of money, and since this happened before, I am worried that this might have been the root of my timing chain skipping problem.
I've heard that a coil on its way out could be responsible or that there is a faulty connection from the coil.
I had a lot of problems with erratic revving above 3000 rpm and other symptoms that appeared to be carburetor related. In the end it was the points condenser that was slightly loose and causing ignition problems.
Definitely worthwhile checking the low voltage ignition circuit for any loose wires and it may indeed be time to think about replacing the coil and plug leads. The coil is difficult to check without fancy equipment; sometimes you can "see" bad plug leads by watching the engine run in complete darkness - you'll see the sparks! Easiest, if not cheapest, is to replace them and see if that improves things, but check all the wires and earths first.