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I doubt its temperature, temperature can effect timekeeping but not amplitude that much until you reach temps way lower then 68 . When its on your wrist constantly being wound its at "full wind" and the mainspring is giving the train the most power. However once off the wrist, the mainspring is winding down and the amplitude is dropping rapidly, likely due to a ever weakening mainspring. Sadly on a 7750 to swap a mainspring means taking the watch apart almost fully.
To test this, let the watch sit and go down to room temp, them pick up the watch and quickly manually wind it until you believe its at full wind and put the watch on the tester. My guess is you will observe a low error and a decent amplitude.
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