Possibly a further factor in your speed variations could be that you are flying when and where there may be standing waves.
The record altitude for a glider was 50,699 ft. by Steve Fossett over or near to the Andes in 2006. A powered aircraft would tend to climb (or descend) in these circumstances. With the cruising level held by the A/P, this would tend to vary the speed, or the power needed to hold a set speed.
I think "secondary" standing waves may be found on occasion, some distance downwind from the "primary" one.