Descending air while trying to maintain climb rate? You weren't overloaded, right?
I'm not understanding why you couldn't pitch for 75. Negative climb? Lower the nose enough and you should get it. Unless obstructions or the ground are a factor, you should always pitch for at least Vy, even if the climb rate sucks, if for no other reason than to improve forward visibility.
At 70, you shouldn't have been behind the power curve.
DA of 1000 isn't that significant. You'll get that at Reid today, easily.
If you weren't reporting tach above 2500, I'd suspect an engine problem or carb ice.
Maybe you were running excessively rich due to all the cow farts.