Started up first time this year following an oil change and new NKG plugs and at between 4000-5000 RPM loses all power and takes on a deep rumbling sound. Seems to idle fine up to around the 4000 mark.
Change the plugs again as a first step. Sounds exactly like a fouled plug. First runs of the season are notorious for fouling plugs on these skis. I recommend waiting until after you have run it at least one good time to change the plugs at the start of the season.