Do you do anything during the winter? Do you have a trainer? Rollers?
Lowered HR comes with time man. Are all your rides long and consistent paced? I think if you have a good base built up, try to keep your schedule the same as it is now, but throw some shorter, more intense rides, or even interval training.
Ex: you ride 1.5hrs 5 times a week now. Replace one of those rides with a more intense 45 min ride, or a 1hr fartlek/interval type ride where you are riding hard for a few mins, then lay off and recover.
You need to throw some anerobic workouts, in short. Some of your rides right now may be going anaerobic from the sounds of it. (Fatigue). Without knowing your max HR, its tough to say. When my max was 205, I would keep my aerobic runs between like 155 to 165 to stay aerobic. My resting rate was maybe somewhere between 45 and 47ish.