Hi Michel, we don't set much store by the float test, so I wouldn't worry too much about that. Look for your starter to double in size predictably, and to have a pleasant aroma as a sign of starter maturity. If you make a levain for a recipe you'll want to add it to your bread recipe when it's at its peak of rising, or just beginning to fall. If your starter is behaving unreliably after a period of rising well, this might mean that you need to adjust the ratio of ingredients a bit so that you can replenish your starter closer to the time when it reaches its peak, rather than when it has fallen significantly. Feeding at the peak tends to keep the yeast vigorous, while feeding when the starter has collapsed tends to cause sluggish yeast behavior. Particularly if the weather is getting warmer where you live and your kitchen is also warming up, this can cause fermentation to occur more rapidly and your starter may not be able to sustain its rise between feedings. To slow things down you could try feeding with cooler water and feeding your starter a relatively larger meal. For example, if you normally follow our sourdough starter recipe that calls for a 1:1:1 ratio by weight, try switching to 1:2:2, or even 1:3:3. For example, a 1:2:2 feeding would look like: 1/4 cup (2 ounces, 57g) starter  + 1/2 cup (4 ounces, 113g) water + 1 scant cup (4 ounces, 113g) flour. The goal is to find a twice a day feeding routine when you're feeding at room temperature that allows you to replenish the starter when it's closer to that peak of rising, or just beginning to fall, rather than when it's collapsed. This should result in vigorous rising behavior in a mature starter. I hope this helps! Let us know how it goes. 

Barb

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