Done a london to brighton trip on the weekend

Hey, on saturday I took my ped for a ride from central london to brighton peer using my mates satnav... Was a nice ride avoiding motoways but did cause the 125 skipper a problem.. Its a 4 stroke and ummm.. along one of the A roads I was going 55-60mph to keep up with traffic (bear in mind my top speed is only 65mph and was doing 55-60 for the past 40 miles), and suddenly I lost loads of power and the engine started sputtering so I pulled into the slip road, and i could only do 30mph whilst sputtering away.. So I turned the engine off thinking it was overheated.. Which it was and managed to cruse at 50mph on the dot for the last 15 miles..

The same happened on the return trip but this time only cruising at 50mph, not hammering it at 55-60 all the time.

And now, just normally riding around town it runs sweet but does sometimes sputter or bog faintly then works fine.. Im also struggling to get 60mph topend now. The only problem I can think of is I've damaged the engine? Although it's due for a service in 1000miles..
Can anyone think what the problem could be? any help would be wicked

 

it shouldnt have a problem sitting flat out all day long

its sick get it looked at