I agree that the wind of the bobbin being inconsistent or the check spring being an issue is likely. I'm not familiar with the Tiara, is there an intermediate tensioner? When my thread jumps in and out of this for me, I get what you are talking about--perfect one minute, a mess the next. Also, you said you switched thread, is it possible you need a thread net on this new thread to keep it coming off the cone or spool consistently without pooling or getting hung up? good luck--it's so frustrating to have tension issues.