Starter question and Tartine recipe.

I was recently gifted a sourdough starter that has been in service for many years. I brought it home and had success in the first few loaves I baked the first week of ownership. After about 2 weeks, the starter seemed to start to fizzle out a bit, and I've simply not been getting as robust of a rise as I'd hope for. When I do the recommended float test, I'm not getting a float (which I was originally). 

I'm thinking the problem may be as simple as becoming more consistent with a 24 hour feeding schedule. Sometimes I miss a day here or there. I keep my starter in the oven with the light on. Also worth mentioning is that I live in Colorado at about 5000'.  Any other suggestions? 

Also, I've been using the Tartine Country Loaf recipe, and while I find it to be a wonderful recipe when my starter is cooperating, I also tend to have a difficult time keeping up with all the steps. It requires pretty constant attention all day. Can anyone recommend other sourdough recipes that don't need love every 30 minutes for 4 hours, followed by 6 more hours of things to do!?? I do have a job....

Thanks so much!