About 12 years ago I was flying a Hughes 300 back to South Wales at about 3500 feet when I felt like the machine tipped forward nearly verticaly down I immediatly looked at my Artificial Horizon and VSI which showed normal so I resisted pulling the cyclic back and I sat there in a cold sweat and thought lets get this heli on the ground, then I thought if I do that it will put me off going up there again ( lets face it 3500 is hardly high ) so I carried on with my flight, trying not to look out much and just flying on instuments, every now and again I had the same feeling but I just carried on at the same height eventually the feeling went.
I now regulary fly about at this height and urge my students to do the same, when you get used to being up there it feels quite normal and is a lot safer, on some occasions I have taken the R22 + the R44 to 10,000 feet it feels amazing in the R22 and if you auto down from that height you would not believe how long it takes to get back on terrafirma !!!!!!!