Debbie Hamilton

I love my 2002 SC 430 - have driven it for 20 years. Have had lots of work done on it recently to keep it going another 20 years. It has recently stopped starting about once every three weeks. New battery, turns over strong, but just won't catch. I keep trying about every ten minutes. After about 30 minutes it catches and all is OK. My shop that takes care of my car beautifully can't find anything wrong. Ideas? Thank you, Debbie