L322 gearbox

Launching a problem ro see if anyone can contribute. My 2004 Td6 is strange when cold (or just cool). I back out of the garage after beeing idle for long enough to cool down completely (or oils to settle ), put it in D and...............nothing. Sounds like it's in N. Idling for 30-60 sec then something starts to happen - slowly. And then suddenly it goes like normal. To little or wrong gear oil? Hardly - been changed (and flushed) twice last year. Sticky valve? Possibly. Wrong tuning electronically? Possibly. Broken gearbox? Hope not......

Could be a blocked filter,the boxes I have replaced have had loads of rubbish attached to the filter.