The engine fluctuations are either caused by:
Engine damage - perform engine maintenance, fuel system maintenance, and change oil via the SR22 maintenance window
Corrupted state file induced by previous crash of X-Plane - you can delete the state/record files in X-Plane 11/Output/SR22
Complete CPU saturation, the engine model needs to run at a fixed rate, if the computer can't provide the CPU usage, stutters and fluctuations can occur. This is relatively easy to diagnose, open Task Manager while flying and see if the CPU is at or very close to 100% usage.