Mine does the same thing! I found that if I buy a gallon jug of distilled water, and mix it up right in the jug, it never turns cloudy and I use and reuse and reuse indefinitely and just pour it back in the jug when I'm done. One gallon of distilled water will last seemingly forever- but I usually end up spilling it all (should use a funnel, I guess) before it turns cloudy! The longest I've kept a one gallon jug of star-san around is about 6 months- not bad for a $.99 cent distilled water purchase- and it would be longer if I wasn't sloppy and messy with it.
One great thing I do after I mix up the gallon jug is to put some of it in a spray bottle and I use that to sanitize the hydrometer, and small stuff like that when I keg or bottle. I often just use the spray bottle instead of the jug I have, as it's super convenient.