Open/closed Loop, coolant temp sensor

[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!

My goal is efficient operation and high mpg (don't laugh). I have some perhaps radical ideas for this when I can get normal operation in place.

I have read many of your posts and see decarboning and cleaning the maf sensor as important. Have had good experience decarboning carbed engines by pouring water thru the carb.

Scan Gauge: wont give trip or mpg readings on this disco. Sent back to factory, no change. Have had the folowing codes:
1158, 1138, PO152 P1187: pretty well solve with the new O2 sensors.
PO307: cyl 7 misfire: solved with tank of BG cleaner and high revving engine under load to blow out carbon.
P1316: misfire causing excessive emissions.. I think solved with PO307.

Borrowed LR hand held scanner: all O2 sensors good, got ATPS code???
Tech said do "adaption"

Plug wires: new at 61kmiles, not sure of brand, may be dealer.

Used to get EPA mpg, 14-16, have got as low as 8mpg, recently up to about 12 after above work. Use 87octane gas and 8oz acetone per tank. Run 215/85/16 tires.

Hope I haven't bored but trying to paint a picture.
TIA
KRMusher