"frowning" within a lane

[dkim at unm.edu]

Hello:

I have a problem with an agarose gel.  I am running 3% agarose/0.5 x TBE
gels with a BRL "1 kb ladder" mw marker.  Within the lanes, the bands
appear to "frown".  That is, the middle of the band runs slightly slower
than the edges of the lane.

Is there an explanation for this?  The marker is diluted into a sucrose-
containing loading buffer with bromophenol blue (per Maniatis).

Thanks

Daniel Kim  dkim at triton.unm.edu