I just received and installed an Indivision ECS in my Amiga 500 with TerribleFire 534 installed. Everything seems to work except with the Indivision installed my cursor jumps all over while moving, sometimes duplicating to other parts of the screen. When I switch back to my Denise chip, behavior returns to normal. Mouse cursor moves smoothly with no duplication. But if the Indivision is installed again, back to jumping and duplicating.
this occurs on the vga connection as well as rgb (over rgb4all svideo board) and mono.
Here is video of behavior with the Indivision ECS installed:
And here is the behavior with standard Denise chip reinstalled: