Blobs on 35S sequencing gels

[Dr. M. Rhodes]

Help
We have been carrying out 35S sequebcing reactions and have hit a problem
WE keep getting a "Blob" not a band but definately a blob approx 1cm accross
going across the whole gel where samples are loaded. It appears to be present 
when you just load the pure 35S-dATP. 
Could it be 
a) the gel we use National Electrophoresis Rapid gel
b) heating at 75 degrees in formamide
c) anything
I have nether encountered anything like this before so if anyone has any suggestions 
I would be only to pleased to hear them as thr "blob" is ruining our results

thanks in advance 
Michael Rhodes
email: mrhodes at crc.ac.uk