Turing T400 intermittently recognising second monitor

I have a Win 11 dual monitor setup (Asus Pro Art + Samsung SyncMaster) Asus is primary display. Recently it has developed problems recognising the Asus in spite of doing a clean re-install of the Nvidia driver.

It only intermittently recognises two monitors but will sometimes belatedly (after 3-4 mins) recognise the Asus

Windows display setup tells me I only have 1 monitor as does Nvidia control panel when the problem ocurrs

Setup has previously worked perfectly for several months