1992 Volkswagen GTI starting issue/smoke/idle problem

I have had this car for about 4 days and when I first got it, it ran great and then started having trouble turning over the engine was cranking but nothing so we figured it was a fuel issue and I changed the fuel filter that has seemed to help thus far but now I am starting to get alittle bit of white smoke and it is moist after changing the fuel filter I had a friend rev the engine and it blew out some black smoke but I think that was air in the lines from where I took off the lines I drove it for awhile but sometimes it will smoke again not much though just seems to come and go this is the white smoke mind u. Also when I start it up seems to idle fine and all that but after alittle bit of driving it will idle rough and lets say I put my foot on the clutch to goto stop and the rpms will dive and almost kill out the car I had a 98 golf that did the same thing but I dont remember what caused that issue or how I fixed it cause at one point I did fix it. But other than that the car runs great drives perfectly!