Here's a secret I'm going to share, from my arsenal of aggravation working with UL fabrics:
On feather weight fabrics, using a wide aperture throat plate (i.e. the ones with oval needle portals), is one of the major contributors to a wide variety of problems, which can appear to be related to tension or timing. The issue is that the portal is wide enough that the needle causes the fabric to press into the opening, interfering with uptake. Bunched up bottom thread, broken thread, fabric puckering, etc, can all be caused by this problem.
The solution, and the trick to beautiful stitches on these types of fabrics is a straight stitch/top stitch only style throat plate, which has a small round hole, 1mm or so in diameter.
I've never seen this mentioned before, I'm going to assume it's because most people don't have multiple plates for their zig-zag machines, and the people that have straight stitch only machines never have as many problems.
I constantly see people refer to the lower thread bunching as being an issue with tension, personally, I've never seen a tension setting with any of my machines where bunching was caused with a straight stitch plate. However, I can instantly duplicate it with the zig-zag plate.
The only other issues I've seen cause this problem are in-proper bobbin windings, or burrs or other problems on the shuttle hook.
If you can find a straight stitch throat plate for your zig-zag machine, buy it, or find an old machine that's straight stitch only for sewing Momentum/Intrepid type fabrics, and save the zig-zagger for when you need it's particular talents.