Probably a good thing to do first would be to clean the battery clamps and terminals (both contact faces) with a wire brush or sandpaper. I've had cases of 'invisible corrision' that caused a mysterious voltage drop!
I wouldn't worry about the CCA. Firstly, testing it must have been interesting... that's a lot of amps for a tester to draw! Surely it is just an estimate?
Secondly, the 903cc starter motor is only 0.8kW. Do the maths: that's 800W / 12V = 67 amps, assuming complete efficiency. Add on a 10% or so 'fudge factor' and get about 75A maximum draw - so well within your 189 reading.
Bubbling - personally, I regard this as a good sign that charging is taking place! Provided the battery is not a completely-sealed type (they are never supposed to bubble!), the bubbling shows that the cell is nearing a full charge. I tend to leave the battery bubbling away for about six hours, hoping by then that any sulphation has cleared from the plates. Perhaps others will disagree. I don't see any harm occurring, as long as the level is correct (get some distilled water if not, don't use tap water!)
The worst thing that happens to batteries is not over-charging, but when they sit in a partly-charged condition for one or two months. If this has ever happened to yours, be prepared to kiss it goodbye
Lead sulphate builds up on the plates as a battery discharges - and the electrolyte goes from H2SO4 to H2O. The process should be reversible, but after a time the lead sulphate 'cakes' on the plates and cannot be broken down by recharging. Thus, the electrolyte is weaker (and so raises the cell's 'resistance') and the 'active lead' of the plates is masked by the lead sulphate. A 'vigorous' recharging from an external charger is all you can try, really... though I have read that a long, slow charge (say, 1A or less for 15 hours) is best. Maybe try that first, and up the current if there's still no progress!
Thirdly, when you say 'struggling to start', what is happening exactly? Is the engine cranking over sluggishly? If so, check what is happening to the voltage while cranking. It sounds like it must be dropping to cause the stereo to cut off: trouble is, different stereos will be sensitive to varying degrees, and as Luke says, you should check the earth (also try running the earth wire from the stereo to one of the heater mounting nuts...)
Before you buy a new battery, try a battery from another car. If there is little difference, perhaps your starter motor needs a clean/brush replacement. If the starter is sick, it will draw more amps than necessary...
If the engine is turning over normally, I think you should check and probably replace the points first, then the spark plugs (or adjust the gap downwards a little). Finally, experiment with pumping the accelerator a couple of times first - perhaps the engine is not getting enough fuel to start in the colder temperatures. But my bet is that the ignition system is weak!
BTW, as a quick check for your alternator, check the battery voltage with the engine off - ideally 12.3V. Then with the engine running, check the voltage again - ideally 14.2V but anyway at least 13V. Switch on high-beam headlights, rear window heater, heater fan, etc. and check battery voltage again with engine still running. The voltage should still be over the reading it was with the engine off - if not, then clearly the alternator is not keeping pace! (Sorry this sounds so elementary, it's just that modern-car voltage rules don't really apply to the Uno, so you need to go back to the basics!)
-Alex
