It looks like your top tension is intermittently changing which would suggest to me that the top thread might be jumping out of the tension discs and back in again. Check your thread path to see if the thread is getting caught up on anything to cause this and that your thread is flossed securely in the tension discs and that there isn't fuzz in them to give the bad tension. Do you use a thread net so your thread doesn't inadvertently slip down the cone and get caught on the underside of your cone? Good luck and hope you have it solved. Tension issues can be a bear and are certainly the bane of my existence some days.