I've had the following experience twice now in the last two months, both on 400 NM cross-countries, both in the same plane, and both in clear skies at 8,500 feet. The first time was more severe and I was sure it was a mountain wave because the winds aloft were stronger and I was on the downwind side of a mountain range. (I also heard another pilot tell ATC about downdraft activity.) But after my experience on Thursday, I'm not so sure.
On Thursday, I was cruising along straight and level at 8,500 feet in our Tiger (a fixed-pitch, 180hp, single-engine plane) and indicating maybe 125 knots. I was running at full throttle (75% at that temp/altitude), and leaned about 50 degrees rich of peak. My altitude started to drop a little, so I brought the nose up a little. Pretty soon I was indicating only 100-105 knots to hold altitude. RPMs were down to 2300 or so from 2600, but I don't know if that's in part due to the nose-up, climbing attitude I was in....? After a couple minutes the plane started climbing, I brought the nose back down, and the IAS climbed back to where it should be.
Winds aloft were 20-30 knots from the west, but I was just west of NYC, and there aren't really any significant mountains up there to create mountain wave action. Sky was clear and temps were cold.
I just don't have enough experience in cross-country travel in other planes to know if there are periodic downdrafts to cause something like this or if there might be something causing a temporary, partial power loss, like a mag cutting out and back in….? I didn't detect any engine roughness and all gauges were normal (there is no manifold pressure gauge).