P0137 - 97 Blazer 4X4 4.3 vortec

first time here, you all can help. Pulled P0140 and changed the O2 in front of the cat. Now pulling P0137. Truck running rough with no power on first start and sometimes will run great after it warms up (about 20 mins.) but only after it is shut down. I change the motor in the truck about 2 months ago with alot of swearring and bleeding and i have put TOO MUCH money into this truck already, but now it's just a man thing and i'm not going to let it beat me. Kind of like dead batteries in the tv clicker and you just keep pushing the buttons rather then get up and push the button on the tv.