It's a newbie question so many of you will laugh. I have to ask it anyway.

I bought a Nikon F3HP body and eager to try it out I purchased a cheap Nikon series E 28mm F2.8 lens at Henry's for peanuts.
I shot and developed only couple of rolls (and never touched it again) and almost all frames had marks on them. I didn't develop the film myself. So, my questions are:
1) How do I determine what are the origin of these marks?
2) Why do they seem to show on some frames and not on all?
3) I know it's handheld, but is my manual focusing super lousy or what are the other reasons for poor sharpness?

Sorry for the crappy shots but they're good examples of what I'm talking about.

Thank you!