Sequencing problems

[Andrew Cannons]

Having problems with sequencing gels.  This has never happened before, 
but the last 2 gels I've run have "expanded" during the pre-heat and 
run!!  That is the comb is pushed out the top of the gel during the 
pre-heat and after about 30 mins of the run you can see gel coming out.  
Any ideas?  We haven't changed any solutions and have been running these 
gels the same way for years!
Any help gratefully appreciated

Thanks