zzr250 intermittent engine problem need help

Hi I'm new to site. Just got a ZZR250 on the weekend.
If anyone can give me some ideas on where to start trouble shooting it would be great.
My first ride and the bikes engine cut out twice!! Was riding for about 20min before it cut out the first time. I thought it was running out of fuel as it seemed to splutter for about 30 secs to a min before it died. Found the fuel tap and turned it to reserve (ended up being about 7 litres in tank when I filled up). It still would not start. Battery died.... great! I was on a good sized hill (Maquarie pass, Wollongong) so I decided to try and clutch start it. Yeah that worked great, bike worked fine for 5 min, until I caught up to some cars, I backed right off to give myself some space (very low speed and RPM). When I went to take off again, the bike spluttered and died, I couldn't clutch start it. So I rolled down hill for a few minutes. Tried clutch starting it again and it fired. It went fine for the 15min ride home????
I know the previous owner and this is the first time it has played up.
It has been parked outside on and off for a few months (water issues??)
It was very foggy 5 minutes before it died the first time. Could the mist have caused an electrical problem??
Where do I start, or do wait and see if it happens again??
Update (2 days later): I just went for a 1hr ride and no probs. Do I pretend it didn't happen?
If anyone can give me some ideas on where to start trouble shooting it would be great.
My first ride and the bikes engine cut out twice!! Was riding for about 20min before it cut out the first time. I thought it was running out of fuel as it seemed to splutter for about 30 secs to a min before it died. Found the fuel tap and turned it to reserve (ended up being about 7 litres in tank when I filled up). It still would not start. Battery died.... great! I was on a good sized hill (Maquarie pass, Wollongong) so I decided to try and clutch start it. Yeah that worked great, bike worked fine for 5 min, until I caught up to some cars, I backed right off to give myself some space (very low speed and RPM). When I went to take off again, the bike spluttered and died, I couldn't clutch start it. So I rolled down hill for a few minutes. Tried clutch starting it again and it fired. It went fine for the 15min ride home????
I know the previous owner and this is the first time it has played up.
It has been parked outside on and off for a few months (water issues??)
It was very foggy 5 minutes before it died the first time. Could the mist have caused an electrical problem??
Where do I start, or do wait and see if it happens again??
Update (2 days later): I just went for a 1hr ride and no probs. Do I pretend it didn't happen?