I have an M-reg Mk1 mondeo 1.6 zetec and.. incase you missed the title, she pours out white smoke under load and.. she doesn't want to go past 50mph

it's the latter part that's pissing me off if i'm honest!! she accelerates with no problem upto 50 but as soon as i change from 3'rd into 4'th or 5'th, she just won't pull. Any ideas, or is it rebuild time ? she's on 120k.
Scrapping it isn't on the list of things to do - so no silly comments.