Thanks again for all the replies!
I finally replaced the fuel-pump (just to make sure that it could not be the cause) - with no lasting effect. I had enough of it and exchanged the ECU between my cars. I was astonished to find that a 6CU was still installed in the XJS with the intermittent running engine and a 16CU in my backup car. The swap solved the problem right away - it runs again like a dream! i assumed that both my cars had a 16CU - and that was simply not the case. Could have been so easy.......
Best
tfl

 

SWEET.

Another simple repair to a simple car, got to luv it HAHAHA.