Slant 6 power issues

Hey guys. I’m coming to you again to ask for your wisdom on slant six tuning. I’m having power issues. Before, the car would push itself at idle when I put it in gear. Now i have to give her quite a bit of gas to get her to move, but too much throttle at once and she chokes. And when I am cruising down the road, it doesn’t sound like it runs super great. It kinda stutters just a tad. If I adjust the timing at all in either direction it gets worse, so I know it’s not that. I’ve messed with the idle screw. It runs fine at idle, but why doesn’t it make any power anymore? Could it be purely due to the gas as my dad suggests? There’s a little bit of old gas in the tank mixed into all the new gas I put in. Is that the problem?