Problems with Tamiya acrylic paint

Hi, I'm currently brush painting using some Tamiya acrylic paint, and when I apply a second coat, after a few moments the new paint seems to partially melt/soften the first coat of paint. This occurs even when the original coat is fully cured (several days). Usually this is not a major issue, as the original coat drys again as the top coat drys and it all looks fine. However, I want to apply a coat of Tamiya clear acrylic over an original coat of paint. This is a problem as you can actually see the paint underneath the clear acrylic is being attacked and the finish is not very satisfactory. Has any one else had this problem? Any help would be appreciated, thanks.