how can I fix my 2001 megane cabriolet?

The above Megane drives perfectly well until it is at full operating temperature. It will be fine whilst driving normally until it gets into congested stop-start traffic, after about 5 minutes the engine cuts out, it will re start but cuts out again. If then left for about 15-20 mins it will start again without problem and be ok as long as you do not get stuck in congested traffic again. there is a warning light that appears on the dash its an oblong box with a zig-zag line going through the middle and below what appears to be the same a a diesel glow plug symbol i.e. a squiggle. although the car is petrol.

I look forward to your comments,