Had a problem starting the engine when it was cold. It would drag to the point you would think it was not going to start. Did better when it was hot and even heat soaked. I guessed it was the additional friction of being cold. I increased the cable size on the + and - and cleaned all connections. The battery was a 3 months old Diehard with 800 CCA and even though I kept checking and finding the voltage drop down to 9 volts I had trouble believing that was the problem. Took the battery and had it load tested, it had a bad cell. Bought the Odyssey battery below and now the car cranks perfectly, hot or cold. Best $400 dollars I have spent on the car. 1000 CCA at 32*

 

I solved my similar problem with a Northstar AGM and a Powermaster 9510.

 

400 dollars ??? Oh man!

 

I ordered a Powermaster 9000-10 which was supposed to fit but didn't. Starter motor was too close to the block and pan with no way to rotate away. Sent it back and waiting on a refund.
I bet yours spins good with the 9510 and AGM battery

 

Night and day difference between that and the tired Interstate and mongrel Bosch starter combo I had in there before. I'm pretty happy.

 

what size engine do you have?

 

G-block 400, bored 0.030.