Chain bounces off drivetrain on hard bumps

[conspiracyx]

I have a GT ZuM with a 38T single chainring front w/ a bashguard, a Shimano CS-HG30-8I 8-speed cassette and what I assume is an 8-speed chain. I have an issue with the the chain bouncing around (both up/down and some side-to-side swing) when I am in the highest gears (11T and 13T). Under some conditions, the chain disengages from the drivetrain near the crankset and falls on the bottom bracket area of the frame. The two most common conditions for this are very bumpy riding surfaces (ex: grass) and when I stop pedaling abruptly after vigorous accelerating (sometimes backpedaling a slight bit).

I considered the problem being too long of a chain, but it came with the bike, I assume the previous owner did not modify the drivetrain, and I also doubt I would be able to return to first gear (32T) if it were any shorter.

Is this common? Does anyone have any idea as to how I can prevent this from happening? I am wondering if there are any sort of "inner bash guards" sold out there that I could mount to the frame side of the chainring so that the chain would have a "side wall" stopping it from sliding out and keep it in track with the help of the real bash guard.