I understand your dilema; at age 21 I was very strong for my size, but it didn't show very much. I was 5-9, weighed 148lbs and was a National Class weight lifter (320lb bench/405lb squat). I had been working out for four years to get to that point, but not gaining much weight during the process. Looking back I realize it was my diet. I simply wasn't taking in enough protein and calories. I was never a big eater back then.
Holy crap! You beast!!!
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"In my experience, if you stay with the essentials you WILL build a repeatable swing undoubtedly. If you can master the Imperatives you have a champion" (Vikram).
The reason you can't sustain the lag is because you are so eager to make the club move fast (a reaction to the intent of "hitting it far"). So on a full shot you throw it away too early, which doesn't happen for your short chip. (bts)