HD-with reference to your comment I am begining to think it is the contrail and not the aircraftproducing this phenomenum(?). Although from where I saw it the aircraft had passed me I dont recall the actual head of the trail diverging, and the apparent angles to either side of track were I am sure large enought o have been noticed on radar.As yousay that was a busy time and the controller wouldnt let one aircraft in a crowded sky zig zag along like it appeared to.
One idea that comes to mind that might fit the picture is say a two or three engine biz jet -tail engines and thus from the ground only one contrail.
A previous poster poster described how the engines could perhaps 'hunt' a bit trying to maintain Mach number -perhaps it was very high and it was unsually cold I am sure -and that produced the blobby -uneven trail effect .
Would it be possible for the occasional extra jet efflux from say one engine to deflect the contrails away to the side slightly where after a while the tip vortices spin them back inwards so that the actual trail is a sort of spiral-the sircraft of course runs pretty much straight and the spirals from the ground looks like a low amplitude sine wave.
Anyway thats my little hypothesis based upon whats been posted by others- any takers?
PB