I'd do some testing that you can reproduce, i.e. shooting a specific target (e.g a resolution test chart of some sort) under specific conditions (e.g. full sunlight).

I'd put the lens on a sturdy tripod, shoot with IS on and IS off, shoot at various apertures, with and without TCs and I'd shoot both with AF and with manual focus using magnified live view. If I had two camera bodies, I'd do the same tests with each body.

Then I'd analyze the results and see if I could tell what set of conditions were responsible for lowered performance and if that lowering of performance appeared to be random or systematic Ii.e. if AF is failing is always failing or if it's failing occasionally and randomly.

There's also a big difference between AF being unreliable, and the inability to ever get a sharp image under any conditions.

I'd also look at the images closely to make sure they were symmetric, i.e. the four corners are all of equal sharpness. If they aren't it suggests a misalignment of the optics.

There's probably nothing you can do yourself. The "... then I broke it in half, it was repaired and again worked fine..." makes me nervous. It's possible that something done in that repair is a problem.

I don't know of any independent repair shop that has the capabilities that Canon does for testing, repairing and aligning long Canon telephoto lenses. There are places like phototech.com in New York but I have no idea how good they are or what equipment they use for lens alignment and testing. I don't know of anyone who specializes in Canon telphotos. KEH.com also have a repair service, but again I've never used them.

If I had a problem, I'd document it as I describe above and send the lens along with the documentation to Canon. They can be difficult to get information from, but it is possible if you persist. They do tend to fix it and and just say "we fixed it". I guess as long as they actually have fixed it, that's all you need to know (though more information on exactly what they fixed would be nice to have).