DIG-labelled RNA probes

[GILLIAN NI BHARLO BOTANY PG]

Hi there!
I have recently started doing northerns using DIG-labelled RNA 
probes, and have noticed some strange binding patterns with some of 
them. The probe appears to encircle the band of interest rather than 
actually binding to it. This particularly seems to be a problem with 
heavily loaded gels - is overloading masking the RNA hybrid from the 
antibody? I hope to use these probes in in-situs and am afraid a 
similar effect there would distort an attempt at localising signals.
I will be exceedingly grateful if anyone can cast some light on this 
one.....!
THanks in advance
Gillian