Bread-Bakers v098.n019.5

Hello all,

I've noticed a problem with the whole wheat bread that I 
regulary make.  It rises and bakes beautifully, up until the 
last half hour of the cycle or so, at which point big air pockets or 
"blisters" develop and the top loses its perfectly rounded shape.  
The top is not totally collapsed, however, just a little dented.  
Anyone know what could be causing this? Thanks!
Josee Olivier
jolivier@autoroute.net