Just to make sure this is a tension issue, have you oiled and "defuzzed" your machine lately? Is the thread correctly threaded? Occasionally, I'll slip up and not pull it through the top tension hard enough for it to correctly thread into the grooves.
What does the stitching look like? Do the threads pull to the top or bottom, does the fabric "wave", etc. I suggest threading different colors to see which thread is doing the wrong thing. It is easy to get them mixed up in the end.
Then, I would start with everything set to 0 or "N". Make small adjustments up and down from there.