is it only when you start- or all over a short seam? are you bringing the bobbin thread to the top and holding it with the needle thread when you start?
quite often rethreading and re (setting) the bobbin will fix this problem- also- when did you change that needle last? is there a bit of lint anywhere (in the bobbin case area???
if the top tension is too loose you might get loops on the bottom-if it is too tight you will get the bottom thread pulled to the top-making dots on the top-
sometimes it is difficult to get the tension even- but it sounds more like you have a threading problem---
start at the thread and rethread- don't tie the thread on and just pull it through- and make sure it is pulled up into the tension disks- sometimes you think it is between them when in fact it is just laying along them.