how to slove this problem?

i have a r reg ford transit .last week it was getting slower and slower i changed fule and air filters .at first it wouldent start because all fule lines wer mixed up .then fule lift pump had to be replaced as it wasent working .now im having trouble starting it starter turns realy slowley as if its dranid then next minite it spins fast then slow again .wen it starts one min it runs well next its chugging