my td6 gearbox

Hi,
my gearbox has been giving me a bit of gip. Reverse out my drive put into drive and nothing for around 30 secs then off we go. it is getting worse as time goes on. i am planning a filter change and oil change to see if it improves. does anyone think this will help or am i better biting the bullet and going for a refurb? land rover say fault codes can bus transfer box timeot. also fault codes for gear monitor 2+3 can anyone shine any lght on how serious we are talking