I know how you feel. There is nothing worse than shooting a good 74 that should be 70 or 71 if you take out 2 **** shots you'd normally pin down no problem!
Scoring those sub-80 rounds sort of tells me it is not a swing mechanics problem like a wrist break or swinging too close to the body at impact, etc.
So, it sounds like your knee flex is not being maintained on certain shots. Thin and then fat ... typical of a change in knee flex.
I have, for many years, struggled with the same bloody problem. I focus on the backswing thinking about swinging around my right knee. I think of it as the 'hub' of the swing ...
I'd start off with this drill:
1. Put a ball under your right heel
2. Push your weight onto the balls of your feet
3. Make certain that your shoulders are over your feet. You can test this by taking a club and hanging it loosely from your shoulder and the club should touch the front of your knee and point at the middle of your feet.
4. Now, swing without losing balance or forcing the ball to move. If it hurts by pushing into your foot - you're not doing to correctly.
Do this say 20-30 times and then take it away and see how you go for 20-30 shots. Do this until your knee maintains its flex angle. Also, the mental thought of swinging around your knee may also help.