I have to think the treadmill motor/speed control might be kicking heavy amounts of induction current or back EMF into your local AC lines. It might help to move something to an outlet that's on a different xformer phase but can't say for sure. If you can't do that, I'd think the simplest solution might be a conditioner such as is commonly sold for some AV applications. APC (American Power Conversion) makes such and Tripplite is another common name in the US that offers a variety of power stabilization products. I'm also thinking it wasn't good to defeat the ground on the treadmill. You could possibly add a couple of filter capacitors. I can't come up with proper values but you'd need one from both the hot and neutral sides going to ground. A google search for controlling back EMF and DC motor induction for ideas more expert than mine.
I recently discovered that it has been my treadmill that has been causing my modem to go bezerk and lose internet connection. Same treadmill was causing static on phone. Bypassing ground (third prong) connection on t'mill fixed that, but DSL interference continues.
Any ideas on how to fix. In UK someone suggests a Tacima Line Conditioner/Filter. What about a USA 110/120 v product?