In my experience intermittent tension issues that seem to have no rhyme or reason, have been caused by:
A blob of lint lodging in the bobbin case tension spring (happens most often when I have cotton thread in the bobbin but I have also had it happen with 100% cotton in the top and Poly in the bobbin. I have also had it happen with flannel backing as flannel tends to shed quite a bit of lint). When this happens, the bobbin tension appears too tight and pulls the top thread down resulting in eyelashes and pokies on the back, the top looks great.
Inconsistent tension on the wound bobbin - with this issue you can have all manner of tension problems rear their ugly head. Can appear on top or bottom. Some prewounds are notorious for having the tension change the last 1/4 of the bobbin.
Top tension check spring needs to be replaced. Like above, all sorts of crazy tension issues on front or back of quilt, loopy stitches (usually on the top but can also happen on the bottom) or what appears to be poor stitch quality that comes and goes with no rhyme or reason.
I can't remember Watson, did you buy this machine new or used? If used, do you have any idea when the check spring (the little "U" shaped spring that comes off of the top tension assembly) was replaced? If you don't know, it might be time to replace that check spring and order a second one to have as backup. I've had check springs last as little as a year and as long as 5. Guess it depends on how many quilts I have done.