I have been seeing this problem for the last 4 years. I fire at cone 04 and also at cone 6. I use the same underglazes for both on red clay and white clay at cone 04, on Bmix5 at cone 6. The cone 6 work just fine but the low fire flake/ shiver at cone 04. Sometimes the underglazes flake off the flat areas on tiles as well as the sharply curved areas like rims and handles. I think it is due to two seperate issues. Firstly the glaze for both temperatures contains Gerstley Borate, more at cone04 than cone 6. GB can help solve crazing issues in a glaze but if added to excess it can cause shivering, the opposite to crazing. If you add to this physical characteristic of the glaze, the underglaze issue of flaking, you can see how the underglaze can be forced off. However, underglaze should be adhered to the clay body by melting to it. Therefore it seems reasonable to assume, since mine work fine at cone 6, that at cone 04 the underglaze is not melting to the clay body. I tried firing to Bisque 01 and the issue mostly disappeared, the deep greens still flaked as if unmelted to the clay. I realized that these two colors were very old pint jars that had settled out repeatedly over the years. I add water and stir them but they settle every time I open the jar. I think that old underglazes may change compositionaly as they are used, heavier particles settle, lighter particles get used up since they are at the top of the jar. I have been guilty of not reading the label instructions, which, clearly state to stir thoroughly before using. To sumarise I think that underglazes should be bisque fired higher to melt them to the clay body. Secondly glazes with GB should be balanced by replacing the GB with Fritt 3195 to reduce the glaze comression on the clay body. Thirdly the underglazes should be stirred vigorously every time used. Old underglazes should be modified by adding fritt 3195 if they seem to be settling out and flaking. Adding a suspending agent to the underglaze should also help with settling issue.