Joe, I seem to remember reading in Philip Eden's 1994 Weatherwise (it's a shame he doesn't post here anymore), the reason why the Jet Stream typicaly meanders in early spring is because the contrast in temperature between the Atlantic & Continental Europe is at its minimum.

Once Continental Europe starts to warm up later in spring, the Atlantic is still relatively cool & takes longer to warm up, this creates a bigger temperature contrast heralding the return of the Westerlies.