Hi there. I have a 300TDI Discovery fitted with ZF automatic transmission. I noticed the engine RPM will drop tremendously i.e. from 3000RPM to 1800RPM when the gear shift from 3rd to 4th. The RPM then slowly increase from 1800RPM to more than 2500RPM then only I can accelerate again. To over come it, I usually manually shift it to 3rd, picks up enough speed and RPM then manually shift it to 4th. This only happens after then engine has warmed up.

Please advise what is wrong. Thank you in advance.