2001 d2 auto no torque converter lockup

Hi all, I hope you are all keeping well.
I'm having an issue with my auto gearbox on my 2001 td5, has 219,000 miles on the clock.
The gearbox was replaced a few months ago with a 2nd hand unit with 100,000 miles on it. All going well though the torque converter only locks sporadically and only first thing in the morning when cold. ( to add it never locked up with the old gearbox in)
The oil and filter was changed when the gearbox was installed and twice since then to make sure all is clean etc.
Changed the transmission oil temp switch which was suggested but hasn't made any difference to the lockup.
While changing the switch I lost very little oil and it wasn't flowing out as others had suggested it would when the switch was removed.
Believing the oil cooler could have been partially blocked from the gunk from the previous gearbox, I replaced with a new oil cooler. Changed oil to correct level etc.
Took her for a test drive and torque converter locked up and work 4/5 times on the run.
Since then it is back to locking up sporadically.
Any help/ ideas at this point would be greatly received.
Apologies for the long post.
TIA
Ally

Hi, first of all you'll have to plug in a compatible diagnostic tool to reset transmission adaptive values to make sure it's not a management issue especially if it made this trick with the old box too but once you are there check the engine management inputs too cos if there are some issues the transmission is mixed up too as the two ECUs are working together to achieve optimal driveability.

untill then run a stall test on it cos if you get "reduced engine power" at least the box is ruled out and the autobox ECU would not activate the lock-up solenoid without a proper input from the engine ECU.

Thank you Sierrafery!, I will look about a diagnostic tool and will do a stall test today, see how that goes.
Thanks again!

UPDATE,
Done the stall test. All within the normal range, 2600-2800rpm.
No luck this far with a diagnostic tool but still working on it.
Thanks!

When you changed the gearbox, did you change the torque converter too ?

No, same torque converter, mechanic seemed to think it was ok.

I would be suspicious of the TC.
The only way to inspect the actual condition of the TC is to cut it open.
Sierrafery is on the nail with inputs to the TCU.
Firstly I would check every sensor, water temp, fuel temp, Maf, Map. These checks although time consuming cost nothing but time.
If all are good then you could be looking at the TC

Thanks DereklyD2, appreciate the advice. The only ones left to check are the Maf and Map sensors as have checked the rest.
The hi/low sensor in the transmission reads the wrong way round according to the nanocom but I've been assured that's normal on a nanocom.
Did have a code that I can't seem to find out what it is. zf2 came up in the d2 autobox function on the nanocom, cleared ok but curious what it could be.
Again thanks for the advice!

Just to confirm the hi low switch the wrong way round is correct, I think its the way nano com reads the voltage I have read an explanation somewhere but can't remember, but it is right.
Sorry not sure what zf2 is.

Thanks for confirming, appreciate that!
May have to start looking for a TC soon.
Will check the MAF and MAP first though.