you've mentioned changing needles...but when did you clean under the bobbin area last?
quite often a piece of lint can cause all kinds of frustration...take the bobbin & case out - give the machine a good cleaning- then rethread- put the bobbin back in & see if that helps. it is recommended to clean at each bobbin change but i know people forget sometimes until they start to have problems..also make sure you are not turning the needle in the wrong direction when putting it in- sometimes having one backwards will do that too.