OT: Necchi sewing machine - HELP!!!!!!!!!!!!

I am having some few problems with my Necchi sewing machine. I had that machine for over 10 yrs. Right now I have tried to tighten/loosen the tension (the bottom thread is not holding the upper thread very good and skip few stitches when I tried to sew the straight line). I have the manual for that machine but nothing seem to help. I was getting ready to sew last week but found out that my son have been playing with the bobbin case (under the sewing machine) and that got rusted. So my husband took to his work and "sandblast" that bobbin case (the one is holding the thread). I have sew the pattern together and then few days later - not doing really good. I have look for them on the website to find more informations about Necchi but don't have them in the model number (3355/3354). I would really appreciates if anyone point me to the right directions. Thanks! Marcie