Well, I'm no expert, but I'll throw in my 2 cents.
I've found that if I let it go too long it gets a bit alcoholic (to my taste, anyway), and maybe that's what you're calling an "off" taste? I let it kefir until it's a bit sour, then I take the grains out and add fruit, and let kefir for about 24 hrs., until it tastes sour-sweet, and then I remove the fruit and refrigerate. Don't know if this helps.
Linda, I'm so glad your kefir grains aren't dead! About the fructose thing, agave nectar is mostly fructose, so maybe if you try that next time it'll work better? It does for me. And I just learned that the light agave nectar doesn't work any better than sugar, so use the amber (looks like honey). Of course, that takes us back to minerals, too. There are minerals, etc. as well as fructose in fruit. It seems to be a combination of factors at work here.
I'm so glad to have this group, so we can share our experiences and learn together!