Defender Td5 running problem

Hi I’m new to this site but need help with my defender td5. It’s been running rough from start up and unless you start it and leave it for 10 minutes to warm up it’s impossible to drive! It ‘chugs’ while puffing our white smoke with no power and not getting over like 10 mph. It can often stall aswell. After driving it will all of a sudden just blow through and go back to normal! Any advice would be appreciated!
Also when it is running properly it hits a lag point at about 2000 revs in all the gears which limits it to about 65mph. Again any advice welcome! Thanks!