If your lights were dimming under an A/C load, the diagnosis would be easy, inductive effect causing voltage drop, but I suspect your problem is the same.
How far from the main panel is the sub? What is the load on the insta-hot? What size wire was run to it? In other words, you've got a voltage drop problem dude, sounds severe. Can be caused by loose connections, check your breakers for heat, and all your terminal connections, if that ain't the cause, you need to check wire sizes and lengths.