I tried looking at the aftertouch strip and cleaning it too, but I realised that if I tore it apart, I'd never be able to reassemble it and that it's not worth the hassle.
Anyway, after cleaning and reassembling it all of the keys I had trouble with ar working great!
Now I have an issue with some keys not willing to turn of after triggering them, but this is a far lesser problem.
There may have been some alcohol, and/or water left in the rubber contacts after cleaning them, because I was in a rush and didn't give them time to dry propery. I hope that it will evaporate or something in time.
Here are some pics from the "disembowelment":
I'm still clueless as to how to get the 1.3 ROM chip and what I would gain from it.
Have there ever been initiatives to create the chips like there have been for K4? I've seen that you can still purchase the updated K4 chips on ebay.