In the red felt photo, is gray the bottom thread and brown the top thread?
What is happening is glitchy tension. The tension is correct for awhile, then the top thread (brown) starts getting pulled to the bottom. I agree with cleaning the entire bobbin area and changing the needle. Be sure to clean any lint out from the feed dogs too.
Not sure what kind of bobbin case this machine has. If it is a removable bobbin case, run your finger around the case and inside the machine, around the needle hole, etc. to see if you can feel a burr. Also feel the old needle for a burr. My guess is that the bottom thread is intermittently catching on something, increasing the tension on the bobbin thread, which in turn pulls the top thread to the bottom. If it's a top-loading bobbin, run your finger around the bobbin area to see if you can feel a burr. Burrs can be smoothed out with the finest sandpaper your hardware store has (emery cloth).
If the top thread was catching intermittently somewhere along the thread path or in the top tension guides, the bottom thread would be showing on the top like that.
The best way to set a balanced tension on a machine is to do it with a zigzag stitch, as the zigzag shows up any imbalances.
On the blue felt picture, is the white thread the bottom thread? If so, either the top tension is too loose or the bottom tension is too tight, as the dark (top) thread is being pulled underneath to make those dots.
Last edited by Prism99; 08-27-2015 at 10:43 AM.