it will probably be your headset
undo the headset and take out both bearing races, soak them in white spirit for a while and give swirl them round a bit to get the dirt off them.
Then get some 3 in 1 oil (or any mechanical lubricant, bike lubes work nice) just dont use wd40. Put a few drops on each bearing race, not too much it will just attract more dirt. put the bearing races back in to the headtube and put the fork back in screw in the heaset cup and the nut .
screw the cup down until the fork fork doesnt wobble at all then tighten the nut against the headset cup while keeping the headset cup in the same place.
slide the bars right down and do up the clamp really tight.
hopefully that works