986 Strange Problem

[krs2230]

I have a 99 Boxster that I have owned for 4 years. The car has been wonderful but I had something very strange happen this weekend that I wanted to share. Car has been running great, stopped at the grocery store for 15 minutes on Saturday and when I came out the car would not start. After a few tries I could tell the battery was not spinning the engine as it should. Came back the next day and installed a new battery. Car would then turn over beautifully but would still NOT start. I had it towed to my mechanic. Two hours later they called and said the car runs fine and started the first time they tried!! I'm perplexed as I don't want this to happen again. Has anyone had a similar experience? Any ideas? Thanks in advance for any response. Kevin