It's been years since your question and I hope you have found relief. I tripped over your question trying to find answers on rib cage spasms myself.

In researching it, it does seem to often be associated with gastro intestinal reflux disorder, so I hope you've seen a GI doc.

In my case this area does not seem to fit. My spasms are not light and are sometimes so severe that the muscle under my right ribcage literally "stands up" - it is visible, like there's a snake in there. The pain is gripping and lasts for minutes, sometimes to abate and come back.

Mine started happening after a bad car accident and is made worse by twisting at the waist or doing anything that involves tummy muscles, like lifting the gargage door overhead.

hi when you find out let me know i have had the same problem for 13 years i get nerve burns at pain treatment center it helps for a while. you may want to check out pain treatment center near you there called nerve burns or you can get ficett joint injection with steroids as long as you don't have austo parossis i don't no how to spell a lott of these words but i hope i helped. doctors can't seem to tell me what wrong with my ribs. so good luck.
tim