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Jim,
I understand not wanting to throw a bunch of money down a rat hole. But a new indicator will be a lot of labor as well. Here are some thoughts:
First, Jim Stack replaced all of the his engine instruments (pressure, temperature, but not tach, MP and fuel flow). He may have some thoughts about how much work it is to do what you are taking on, but he also (obviously) made a similiar decision to yours, so he may be a good reference point. Mr. Stack, if you are reading this, can you comment? Also, remember that Stack has a VERY good relationship with his local FAA, and that has to have made his project easier.
If it were me, I would remove the tach, and take it either to a local instrument shop, for testing, or failing that, I would take it to my avionics shop and ask them to use a pulse generator to test the instrument itself. If the instrument checks out, the problem seems clearly to be in ships wiring, since the chance of both mag sensors going bad at the same time are low.
If the problem can be duplicated on the ground, have you considered just making a small cable to run directly from the tach sensor (mag) to the instrument? If that makes it work, then you KNOW it is a wiring problem. You can also have your avionics guy use a scope to look at the signal at various points, that may give him a clue as to what is going on.
Getting an avionics guy that you know and trust involved with electrical problems can be very helpful. Many (but certainly not all) A&P mechanics HATE electrical issues, and often want to solve them by total reinstallation, rather than tracing down the problem. That may well not be the case with yours, no insult intended, but it does happen.
Here is what I am getting to. If you put in a new instrument, you probably are considering new wire too. If your instrument and sensors happen to be good, then you are throwing money away replacing them, because either way you will be paying the same or more labor to fix the wiring problem, by installing all new wire.
Bottom line, you should be able to get the instrument tested, to determine if it is bad, rather than buying a new one in case it is bad.
But I *do* understand the frustration.
And, in answer to your original question, I have never seen an aftermarket tach in a 337. Does not mean it is not out there, but it may be hard to find. I would look at Ernie Martin's site on parts sources, maybe ask Gmas on the Yahoo list, and also contact the CPA for their sources, and in general do whatever I could to find a replacement tach of the OEM type, rather than do an STC or field approval. There are a lot of 337s out there being parted out, and since tachs fail so rarely, you should be able to find another. If you install it and it does the same thing, then you know you have a wiring problem.
I realize that I am rambling on here, but seven years of Skymaster ownership taught me that the wiring used in these birds was never designed to last this long, and on my airplanes, the best best for any electrical type problem was some sort of wiring issue, rather than a bad major component. Not always, but very, very often.
Sorry to go on at such length...
Kevin
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