Intermitent starting problems

Hi my 1999 206 xsi has been playing up for a few months now and is gradually getting worse, the car sometimes wont start, sometimes it turns over and over then eventually starts and it smells of petrol, the engine management light comes on sometimes, its been into the garage a few times but always starts no problems, I have had the car from new and there has been no issues with it so far. it is the original battery and was wondering if it could be down to that as sometimes when it wont start it flattens the battery until it wont turn over but leave it a few minutes and it seems to be ok and will turn over the engine and start but the radio and clock etc need resetting. hope someone can help.