Help! Open.Closed Loop, Please comment!

[KRMusher]

Hello All,
I have changed both front (pre-cat) O2 sensors and done a 90 k service. Am about to turn 100k miles on this 96 Disco 1 SD, US version.
I have a scan gauge and read the sensors direct. Here is the issue:

Once the engine is completely warmed up, including the exhaust manifolds, it switches to open loop operation. If I turn the engine off 10 seconds it goes back to closed loop but than switches again to open loop when fully warm again. So far I have done all my own work with friend's help.

LR mechanic says: do a 30 minute reset of the "adaption" with the testbook to get the computer back to original specs. I think this is reprograming the PROMS.

LR service writer says: replace coolant temp sensor since it goes to open loop only when fully warm.

Any opinions welcomed!

Already posted longer version under Open/closed Loop Coolant Temp Sensor but nobody responded! PLEASE ANY ONE WITH COMPUTER OR obdII EXPERIENCE LET ME KNOW IF THESE DEALER SUGGESIONS MAKE ANY SENSE.