One of my training captains once told me that "blob contrails" as you so eloquently describe them
are caused as a result of the speed governor mode of the autopilot.If you encounter a varying wind,with a specific speed set on the speed selection mode,the engines will automatically change throttle settings to maintain the selected airspeed as the winds are encountered.thus causing unequal exhaust gas temps as the engine settings are reduced and increased.
i'm not 100% convinced of this explanation,but if anyone has a more authoritative answer please,i'd love to know.I've also seen contrails that seem to have 'knots' in them at regular intervals,but only on one of the individual trails,the other one (in a twin eg) will be straight like normal.
Cheers,Cb