Then the radio just died. Maybe the antenna was to much for that 25
year old Cobra 148 GTL?

If the antenna was correctly tuned it would make no difference what it was
to the radio you were using. Three things spring to mind.

1/ The co-ax was damaged when the antenna fell off the roof in the first
place causing a possible intermittant break of the inner core .. or..
2/ The radio itself has an intermittant fault. Quite possibly dry joints on
the circuit board on a model that old.
3/ The power supply lead you have connected the radio to is dodgy. Try
taking a couple of wires (fused near the battery of course) from the battery
to the CB radio... possibly through a supressor of some kind ... by-passing
any possible faulty wiring in the car itself.

There are plenty of heavy duty magmounts available for the larger mobile
antennas including a tripod type magmount affair with the antenna mounting
on a bracket between the 3. The wind will never blow this off... only thing
is you might have trouble getting it off at night but if you leave the base
on the car there would be no problem.
The surface area of a 5ft firestik is pretty large and suffers from wind
shear much more than a base loaded coil. Unless you have something pretty
severe as a base you will have problems, even with the 4ft version.

Regards,
Graham