Similar problem -harsh down-shift to first from a "slow-roll" after car is driven 30 - 40 miles and then in heavy traffic only at traffic lights etc. No codes ! Does NOT do this when cold or in highway traffic 60MPH plus, only after a drive and then in heavy traffic. I haven't and can't check in the next few days a "heavy foot" 2nd to 1st down-shift but I was inclined to think trans PCS and you seem to feel most likely NOT? I wouldn't say there's a "loss of power" with mine though.
Comments? The car is an '03 L67 SSEi 95K miles and serviced regularly.
I do have an L67 '02 GTP also.