Unfortunately, your spa has biofilm it and until you flush that out you will continue to have water management issues. You can go to my website at www.cwsnaturally.com to find out how to eliminate the biofilm, what biofilm is, and how to keep it from returning. All the scientific research over the last 15 years has identified bacterial biofilm as the cause for over 80% of water contamination issues. (http://www.erc.montana.edu/) This link will take to the University of Montana's website so you can see the impact that biofilm has in our enviornment.
Aschwartz, How in the world do you know that biofilm is the cause of the problems without being there and testing the water? Sure it could be biofilm, it could also be that there isn't enough sanitizer in the water since it is being left for extended periods of time, etc, etc, etc. At least be honest and say that you wonder or that you think it could be biofilm, but to come straight out and say that it is biofilm is misleading. You're simply trying to use scare tactics to sell your product which I find deplorable sir.
As for the original poster of the thread, if you are going to be gone without being able to check your spa on a regular basis, chlorine may not be your best option. You might think about using bromine instead since it stays in the water longer than chlorine. At least this is my understanding after reading many of the threads on this forum. I would also suggest finding a post by Nitro as he has a great post which he links to in the signature of all his posts, about how to balance your water to help prevent PH from swinging from high to low, etc. A lot to digest, but definitely worth the read.