For what it's worth, I mash a minimum of 90 minutes, even at a higher temp. I just seem to get better results that way...
However, I agree with the others; dry spots are probably the problem. And since you mention these dry spots, I'm wondering how thick your mash is.... I mash at 1 1/3 quarts of water per 1lb of grain- makes for easy stirring, easy temp control, and an enzyme-friendy environment.