When the lock up clutch engages at that speed, it should be under computer controlled slip in order to dampen vibrations. It is possible that is is fully locking for some reason. Slip at that speed should be around 60 rpms or so.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record