I just switched over to using a sodium borate buffer for my DNA electrophoresis instead of TBE. I did this because SB is faster, cooler, and cheaper than TBE or TAE. The problem is that my DNA bands from PCR reactions or restriction digests tend to run a little bit higher than my DNA ladder, which can make it hard to determine their exact size. From 100-300 bp everything lines up perfectly, but as the sizes increase the bands become shifted upward relative to the ladder. For example, I have a verified 380 bp PCR product that appears on the gel at ~420 bp.

What could be causing this? I have tried doubling the concentration of my SB buffer but it doesn't seem to help much.

For reference, I am using the GeneRuler 100bp Plus DNA ladder and gels made with my SB buffer and 1.5-2.5% GeneMate LE Quick Dissolve Agarose

My 20X SB buffer was made by adding 45g boric acid and 8g NaOH up to 1L of water with a final pH of 8.3. The 1X running solution has a concentration of 10 mM sodium.