Can shaft question

I am replacing a timing belt on a 99 galant 2.4 SOHC and
with the crank at TDC I rotate the cam sprocket to line up
marks and it turns but feels stiff to me, also it seems to
have resistance then all of the sudden frees up and jumps
forward
I am not that experienced with this and need to know how
the cam should feel when turning and if this could be a
symptom of something wrong in the top end