I have two machines; one circa 2004 and one circa 1945. I use the 1945 machine the most, I just like it better. However, on both machines I use the same method for my bobbin tension.
I wind the bobbin and then I hold the thread, if the bobbin falls on it's own it is too loose, when I give it a little bounce and it drops a few inches (6 or so) it is just right. Clumping, knots or a mass of thread on the bottom is usually too little tension, but it could also be the speed the fabric is going through the dogs.
If you don't have a walker foot then you need to 'slightly' pull the fabric throught the dogs to keep it from bunching.
However, needle size, thread size and speed are easy to deal with once you have the bobbin tension set properly, that is the main thing.
Unless the timing is off, I think this will help your situation the most.