[sciguy125]

I figured it out! (mostly...)

It was indeed a ground problem. None of the outlets in the house are grounded. I know that because I personally replaced all of the old 2 prong outlets with GFCI about 2 months ago.

I discovered, using my meter, that the outlets in the garage are properly grounded. (Acutally, I guess it only proves that ground and neutral are connected at some point, not that the system is properly grounded...) My mistake was thinking that the distortion I was seeing in the line was the cause of the problem. So, when I was trying to find what was causing it, I only looked at my scope, not the problemed circuit. After using an outlet in the garage to test my circuit (with a real power supply rather than the transformer...I can't find it right now) it works fine. Not surprisingly, the distortion in the line is still there. So, that distortion isn't the problem, the ground is.

So, I've figured out that to work, all of my equipment needs earth ground. Using a 2 to 3 prong adapter (isolating ground) makes it mess up again. Why is this? I would have thought the opposite would be true.