not necassarily true (but highly possible)
The rev counter is an analogue device, directly measuring engine rpm via current pulses to the coil (or distributor pulses on later tachos), if the fault were on the high tension side this would cause the engine to falter/stumble which in turn would be picked up by the tacho possible showing up as discribed, (try pulling one plug lead on and off at a few thousand rpm and watch the tacho dance)
There's a lot of talk of the tacho possibly causing this fault but I'm with Mark (260TT) on this one, the tacho is a non invasive device, measuring a result (dont blame the messenger....the tacho being the messanger) and is unlikley to create the effect unless the continuity of the coil voltage (which on early tachos passes through the coil) is interupted by a bad connection (30 years of oxidisation).
Think Occam's Razor (and no before any one says it, Gillet don't make one)
He still needs to do a plug chop first to see what that tells us.
"The explanation of any phenomenon should make as few assumptions as possible"
In other words, when your done screwing around its nearly always the simplest thing thats at fault, or as Homer Simpson would say...."Dohh!!"
Failing that then its time to perform that age old ritual that most Z owners fear, loath and detest.....the opening of the wallet ceremony.......go buy your self some rolling road time, and kill 2 birds with 1 stone, diagnose the problem and check your power output at the same time..