Underthread Going Crazy

Question

Hello,I've been having a recurring issue with my sewing machine. Every few pieces it decides that it wants to throw a tantrum.The upper threads will start skipping and suddenly the fabric is tugged. Upon pulling out the sewn fabric the usual result is a knotted mess of threads on the underside. Tugging on this thread mass usually loosens the whole piece and it comes undone. When it doesn't come undone the sewing is still weak and a horrid mess.I've tried what I think is adjusting the pressure and such, but I'm really just guessing here. Any tips? I'm in desperate need to get this sewing machine running asap.Thank you in advance.