Sorry, I haven't had any costochondritis experience - the only thing that comes to mind is this -
Are you perhaps moving or sleeping in unfamiliar ways due to the neurostimulator?
Hopefully your Dr will be able to help you brainstorm about
what this is and how to treat it.
Lynnwood, Lupus & Sjogren's Moderator, Dx: 2000DIAGNOSING LUPUS & HW's LUPUS 101
"Life is far too important to be taken seriously." - Oscar Wilde