Logism: Add/Subtract 4-bit Oscillation Apparent Problem
Why do you feed the carry out back around to the carry in when you're subtracting? That doesn't seem right, and that's the feedback path that's causing the "oscillation" (which basically means that the simulator can't find a self-consistent output state).

To negate a number, you need to invert all of its bits &mdash; which is what I assume the upper-left block is doing &mdash; and then add one to it, which is normally done by setting the carry-in of the adder to 1 &mdash; unconditionally. There's no reason to have the AND gate or the connection from carry-out.