I've had some wildly inaccurate speed and percent grade readings too, usually while riding through forested areas.

On yesterday's ride I noticed the Garmin behaved itself a bit better. It could be because the camera was on the head tube, not right next to the Garmin on the handlebars where it could be a potential source of electrical interference. I'm not positive that a camera can cause that type of interference, but I have seen the percent grade readings go absolutely nuts at the beginning of a ride when there was a camera mounted right next to the Garmin. Also, electronics can cause interference to other electronics. That's the reason for the FCC warnings in the owner's manuals of just about every electronic device out there.

The only other way I've heard of for a cyclist to measure grade seems a bit cumbersome-use a plumb bob or some other crude homemade device.

I have seen pictures of a simple non-electronic inclinometer that mounts on a bike, but I have no idea who makes it or where you can buy it.