The Linkage program is showing a really weird bug.
In the picture, it is obvious that the dotted line that is the arc between two connectors A & C, is not passing through the sliding connector D. This gets worse with time until the triangle shaped link is nowhere near its correct location.
The simulator is never using the previous location of a connector to determine the new location. It always builds the frame of animation using the original locations of things. That makes this a really weird bug. All I can imagine is that there is some aspect of the mechanism that actually is using a previous position of some connectors for some aspect of the calculations. Maybe I’m using the wrong data for some length calculation somewhere.
Anyhow, this is only a problem when the mechanism runs for a while. So far, I have only testing this type of mechanism, and the problem might not happen when the sliding connectors are on the moving link and the arc is grounded. The simulator runs different code that that different type of connection.