Small transport pilot reported experiencing mountain waves and moderate turbulence during cruise. ATC cleared the flight to a lower altitude and the remainder of the flight was uneventful.

Narrative

In cruise at FL430; was in light chop (turbulence) westbound; shortly before reaching TABLL waypoint. Then experienced sharp vertical accelerations and rolls (up to 10 degrees); possible mountain wave but this was unlike previous waves I have felt; motions were sharp and irregular. 'Felt' turbulence was moderate. The autopilot could not hold altitude and was deviating +/- 200 feet from assigned FL430. Then the autopilot disconnected. I called Salt Lake ATC and told them I could not hold altitude; was in a wave or other tubulence; and the autopilot had disconnected. The controller asked if I wanted a block altitude FL410 to FL450; I replied yes; and was subsequently given the clearance. I hand-flew for about 5-6 minutes; and then the autopilot re-engaged as the accelerations-turbulence subsided. The controller offered to ask other aircraft about their flight conditions; looking for a smoother altitude; which I gratefully accepted. They came back and offered FL400; we descended; air was smoother; and the remainder of the flight was uneventful.I am in contact with Garmin to download and review the autopilot data; to see if there is anything to learn. I am grateful for the controller's excellent assistance.