I have tried switching tfi modules. from a new msd to a known working used motorcraft.It's ok they are 2 separate signals, ECU gets the pip, tach gets the negative side of the coil. I assume you have swapped the tfi to try a new one right? The tach is seeing the coil drop...
Try this pull the spout and set the timing locked at 26* see if it still breaks up.
I fixed time at 26 and ran with spout disconnected using the accel coil. With fixed time the problem went away completely. It is something in the pip, tfi, or connection. But I'm not sure why it doesn't exist with the msd coil.