There are a couiple of devices that keep a mainspring in place. One is a bevel on the upper leaf that fits in a corresponding recess in the underside of the bolster of the lock plate near the drum/snail.
The second is a flattened end of the mainspring that has a screw that secures it to the plate. The CVA must have something like this...
Sounds like the hole for the mainspring's tit is too large and it allows the spring to rock a bit, and work its way oout. You could countersind both sides of the hole, fill it with a nail, and redrill it the proper size. that would go a long way to helping the issue. Also, measure the pin at the spring and at the outboard end. If it is tapered, that will compound your problem. The fix is to file it into a parallel pin/tit.
Another way to remedy the tapered pin/tit, is to measure it's minor dimension, drill a hole into the end of a piece of steel rod the correct size, and cut teeth with a hacksaw and files to make a cutter. If you use mild steel like a nail, case harden the cutter. If you file it out of drill rod, harden and temper it by heating it cherry red, quench it in light oil, polish it up and draw the temper to about straw or brown. Clamp the spring in a solid vise, mount the cutter in your drill press and bring the cutter down on slow speed to make it a parallel pin. Easy to do.
