EDIT: I found the problem: with these values cut-off freq is 33MHz and the signal is only 1MHz, duh.
Please have a look at my very simple schematic
The square is rock steady, but after C1 all the fluctuation begins, but I'm expecting a stable amplitude. I even switched to battery power to eliminate any potential noise from the power supply, but the problem persists. I'm not using a breadboard, and wires and component legs are max 20mm (3/4 inch).
It's probably a normal occurrence with hi-pass, but I have no experience with it, any ideas or suggestions?