...It was pretty routine--suddenly I'd have sharp spikes in SWR and drop in signal strength, on and off, or mostly on. When that happened, I'd take off the lip mount, rub off the corrosion, give things a shot of WD-40, and put the lip mount back on, nice and tight. That would solve it for the next few weeks.
The same applies where the antenna screws into the mount - take it out, clean out any corrosion,
put some No-Al-Ox or Ox-Gard on the threads, and reassemble it.
Most likely the problem is an intermittent, either open or short, in the mount somewhere. It might
be that the antenna just isn't screwed in tight enough. Could be some corrosion or an intermittent
connection inside the mount, etc. Could even be on the antenna at the point where the wire
attaches to the bottom ferrule. You can test it by wiggling the antenna while you are parked to
see if you can reproduce the problem.
If it is the antenna, you can try removing some of the heat shrink tubing at the base and repairing
the connection (which might require a solder lug if you can't make a good connection to the ferrule.)
Otherwise look for corrosion, a broken wire or stray braid inside the mount that is causing an open
or short circuit. Before disassembling the mount, you might try a different mobile whip (perhaps a
CB whip or whatever else you have handy) to confirm that it is the mount and not the antenna.
It also depends where you have the mount installed - if it is on a small trunk lid, the problem could
be intermittent connection though the hinges as the trunk lid is torqued back and forth by the
whip waving about. I wouldn't think that this would cause total dropout of the signal, but you do
need a good solid, reliable ground connection from the lid to the frame of the car for reasonable
(If the audio is what is cutting in and out, it could be an intermittent speaker lead as well!)
The problem with mag mounts on HF is that I haven't yet seen one with enough surface area to
provide a low capacitive reactance between the mount and the car though the paint. On 2m
it takes around 120pf to reduce the reactance to 5 ohms, and with a quarter wave whip your
antenna resistance will be perhaps 35 ohms. A 10m mobile whip would require 5 times the
surface area to get the same reactance, but the antenna resistance is also going to be lower
if you aren't using a full size quarter wave whip. While reactance isn't the same as resistance,
it does point out that a mag mount doesn't just perform a mechanical function.
Mobile operation puts a lot of mechanical stress on the mount, especially as antennas get
longer and/or heavier. Any such mount requires regular inspection, and occasional repair
and/or replacement. Insulators crack, screws come loose, and corrosion gets in all sorts
of places (especially where you get moisture.) Looks like a good time to learn how to take
your mount apart and check it out.