I think jump squats would be number 1. If you can explosively fire the left quadriceps in the downswing you probably can generate a good amount of power. Additionally, plyometric push-ups and explosive pull-ups would probably be close seconds.
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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)