I would take a few minutes and check the power steering pump. My original one started leaking and I was chasing a similar issue. If I accelerated hard and backed off up to about 70mph the steering wheel would shake until I slowed down under 50mph or less and then it would settle. Went through three sets of wheels and tires chasing it down. When I cracked open the old pump there was some wear in the vanes, my guess is that at some point the fluid would get pinned to the back of the reservoir allowing some air into the line. New pump cleared up much of the issue. Every once in a while it appears again, but nothing close to what it was.