[Dr J]

I have an LM2 with a working (sort-of) tach signal. The problem is that it starts to cut out and becomes flat. The tach signal comes from an HPX which sends a 5V square wave. That signal goes to my DWUR and the LM2. It is connected to the car's tach through a transistor that isolates the signal and provides enough current to drive the car's tach. The problem has gotten progressively worse. If it were a ground problem, I would expect the other signals to also cut out. Anyone experienced this before?

Here is a picture.