i'd be willing to bet a filter capacitor failed or is failing.
here's one thing you can do.. hit radio shack.. go to their parts section.. grab a pack that has a big choke init.. coarse wire.. might even be called a filter choke.. who nows these days.
grab 2 large capacitors.. like 4700 to 6800 uf.. get ones with a voltage range of 16v at least..
wire up a simple pi filter circuit. cap + leg to one side of choke other cap + leg to other side of choke.. that leaves the neg legs.. tie them.
now on yer radio.. find the power lead.. break it.. put the choke inline with hot. and tie the neg legs to radio box metal . if they have any torrid snap ons.. grab a couple and pop them ont he incoming power and the clock power leads.. give em a few wraps of the wire.
some people also will take a lage value cap.. same as above and tie it right between alt charge stud and alt frame.. usually with a ceramic disc cap parralleled with it.
IE.. I think you may be getting alt noise in...
and I'd be monitoring your alt too.. :)