Trouble with Free Chlorine Values

[scorpionc53]

Good evening, I'm having a major problem with my free chlorine and wanted to come here before I give up and drain the thing. I've had this hot tub for about 2 months now and I was able to easily get my water chemistry all set. 

History: I am using the Frog system which is built into my hot springs. I tossed in some chlorine before taking a week vacation last week and now I'm home but can't seem to level it out. I noticed that Frog had emptied at some point during vacation and there was a slight BO smell to the water, but was clear and I did not see any buildup on the walls.

I first noticed that my pH and alkalinity were extremely low. So I successfully brought those up to the proper values. My current Frog is empty for the Chlorine part, so I have more on order and have been supplementing with DiChlor. 

2 nights ago I did a shock treatment to get my values set before taking a dip and noticed that my FC was 0. So I added a batch of dichlor and called it a night. I also soaked my filter in a solution overnight and made sure I rinsed it well before re-installing the next day.

Last night, I went to take a dip and again, noticed the FC tanked to 0. This time, I did a super Chlor and brought the FC value up to 10ppm. 

Tonight, I came back for a dip and again, the FC was 0. My Total Chlorine is about 3ppm, Alkalinity is around 100, pH appears to be about 7.4, and hardness is around 250. What the heck is going on?

I just did a super super chlorinate and brought my FC up to 20ppm and my TC was over the scale. 

Anyone experience this? I failed chemistry twice in college, but I'm excellent at following directions.

My spa was purchased brand new, is a Hot Springs Relay, holds 345 gallons, and I keep it around 103 degrees.

 

What can I do to fix this chemistry without draining, or is a drain the only way?
Also, maybe a newb question, but what can I do to prevent this from happening again? Do I need to shock or chlorinate after each use?  My current routine right now is check chemistry every few days since I think I can trust the Frog system, and before vacation, that was working well. Every few times I'd use the tub I'd toss in a shock treatment about 20 minutes before getting in.