Deformation of Sequencing Gel

[John Altman]

The other day, I ran a 6% glycerol tolerant gel (USB) with an acetate
gradient.
I attempted to do two loadings, but when it came time to load the second
set,
the top of the gel had become seriously deformed.  I tried to smooth it
out with the back edge of a comb, but all of the sequences in this
region of the gel were extremely distorted.  

I ran the gel at 75W in a BRL S2 apparatus.  The top buffer was 0.5 X,
while
the bottom was 1 X + 1/2 volume 3 M NaAc.  The top of the gel got quite
warm, but was not any where near a temperature that would crack the plates.
The acrylamide stock was quite old (about a year), but I've had no problems

with this stock when running single loadings (run time of about 2 hours).
I always start with solid, ultrapure urea everytime I pour a gel.

Has anyone experience a similar problem and arrived at a solution?  Today
I'm trying again with a brand new acrylamide stock, but I thought I'd
check for a wider range of experience.

Thanks in advance,

John Altman