er..... if you can't go down the line or prevent yourself from nosediving, you probably aren't an intermediate. semantics aside, I was right where you are not too long ago - - still am working on that stuff -- and really trying to get my bottom turn now (tips anyone?).
I'm curious what some of the more experienced guys say...but I hope some of this helps.
To go down the line, pay attention to the wave. If you have a good idea which way its breaking, point your board in that direction as you paddle and take off. It helps to watch the waves from the shore and figure out how they are breaking so you have a better idea while you are out there. Sometimes, you're just going to get a wave that closes out so you cant really get down the line.
For the nosedive, it could be that you are taking off too late. If you take off while the wave has already started to break on top of you, its going to pick up the back of your board and throw it "over the falls" -- pushing your nose down. It also could be that you're putting too much weight on your front foot, or that your weight is centered too far forward on the board. Sometimes I can stop myself from nosediving by stomping my back foot. Experiment with how you center yourself on the board.
how big are you? Riding a 5'6" board could be an issue.
I had that problem, (still do sometimes) but I got this tip from a guy in a line up i was in and it radically changed how I surf... as in i surf better... he told me to:
get higher on my board ( i was so scared of pearling that id hang my feet off the back of my 6'4 quad) and ARCH my back so that my weight was shifted foward but still centered in the sweet spot on the board.
he told me to do this and now its effortless for me to get into the wave...
Dont be like me and get spooked about pearling... it takes forever to break that habit.
as for going down the line, when you find a method to do it quickly and consistently then let me know... i am late.
good luck these things take time... Im in a similar boat with you but i practice and get hurt (my mother bought me a loaded first aid kit for my b-day), rinse and repeat. (and also i highly suggest that you practice popping up with your surfboard on your bed, its the quickest way to find out if your too high up on the board and it really helps buid tone)