I have a 1996 F-350 the voltage started swinging.I notice it on the dash gauge. Had the altenator tested it tested o.k. I checked all the wiring connections they were o.k. sometimes the voltage swings to where the battery light starts blinking on and off. Then there times the voltage stays steady. Any ideas! Thanks

Having rebuilt many alternators, I suspect the regulator/brush holder assembly needs replacing. It's very easy to do:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-13.html#post2518522

The brushes are likely worn down too far, causing intermittent voltage. On run-of-the-mill tests at auto parts stores, this may not show up. Depending upon your engine, it may be AutoZone Part No. VR906, $69.99. Will only take several minutes to replace.

Personally, I would use the techniques discussed in the above link to completely check out and rebuild your alternator.