Terry,
I would completely agree with Ralph on this one as the search will bring up many options for a solution. His sugestion of the capacitor drying up and changing value is also a very common issue with old caps. I would replace all the caps at one time.
Now I have also been working to bring the old into the new technoligy and I found a great circut design on the web that works great with about any type of input. The mod was designed by Steve Maas of Long Beach Cal.
http://www.nonlintec.com/sprite/Sprite_ ... c_Tach.pdfI have built three of these to date and they work very well, one running points and the other two run on a Pertronix. The nice thing is he also provides and easy circut for calibration if you wish.
Mark