If the looping is happening on the underside, it is your top tension that is to blame, conversely looping on top is bobbin tension.........take a look HERE for detail on cleaning lint buildup.
Top tension should be set around the middle of the range, usually 4 - 6.
I would not advise playing with bobbin tensions, however, you can take the needle plate off to clean the hook race area (where bobbin case sits) ...this is good housekeeping, my wife does this every time she replaces the bobbin....just take it out and clean the bobbin case and the fixed metal hook race with a small brush to remove lint, if there appears to be fine dirty deposits, a cloth or cotton bud (Q tip) moistened (not soaked) with methylated spirit will do the trick.
Bargainbox
OK, it seems likely that you will need to adjust the bobbin case.......first check there is no lint trapped in the metal spring where the thread is tensioned. Drop-in Bobbin case will look similar to this image and the tension screw is in the middle of the metalwork... 
...the other screw at one end is holding it all together, so beware....it is not a tragedy to undo the whole lot and clean it, but very gingerly and lay the bits out in sequence and orientation, or you risk tearing your hair out !
When you are certain there's no trapped lint in top tension or bobbin, set the top tension to 4 and the bobbin tension to a point where you just begin to feel resistance....test and adjust the bobbin tension screw very little at a time, perhaps 1/16 of a turn.
Try using good quality thread of contrasting colours so you can more easily spot the changes.
You may find you are playing with this balance for some little while and if you are putting the needleplate on and off each time begin to think it cannot be correct to do this.....BUT....it is, and eventually, you do get a "feel" for the correct tension and then it happens quite quickly.....as a user you won't be doing it very often unless there is lint built up (or small hands at work)
Bargain Box
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Thanks for the above solution(Jan 27th). My bobbin tension was a mess and I simply took your advice and adjusted the top tension and now it's perfect.
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...the other screw at one end is holding it all together, so beware....it is not a tragedy to undo the whole lot and clean it, but very gingerly and lay the bits out in sequence and orientation, or you risk tearing your hair out !
I had my top tension at 6. I raised and lowered it and the bobbin sewing was different but not better.
I adjusted the tension on top and sewed for a while and the bobbin sewing smoothed out. As I kept working, the thread kept breaking so I eased back on the tension. But it still kept breaking. When I got back to #4, the bobbin was messed up again. So I took the needle out, reinserted, rethreaded, etc. I pushed the tension up again and it is working okay now. A friend told me to possibly replace the needle, that often they are "too sharp", even when they are new. So I'll work with it for a while as it is. Then replace if necessary. The machine seems to have a mind of its own. Don't think I'd ever get another Kenmore.
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