Growing power loss after 30s top speed since service on100cc

[antandcharmi]

Hi,
I am a new biker riding an automatic gear Peugeot Kisbee 100cc scooter since Nov 2014, done 5,000 miles, and need help for a very strange problem.
The bike was fine until 2 weeks ago when it had a service. My next two short trips were in heavy rain and I did not notice a problem, perhaps due to riding slower than usual.
But on every ride since then, after riding at top speed (55 mph) for about 30 seconds, the power starts pulsing.
It will drop for a second (just enough to cause me to feel the deceleration), come back, then after a short while drop again, come back, and do this increasingly often and for longer, until it is in lower power all the time. The engine is running sweetly the whole time and sounds fine to me. It is as though I were riding through patches of gravel or the engine was being gently squeezed. After about 60 seconds my top speed is down to 45 mph and stays there.
I use the bike to commute and the trip is a sandwich of 10 minutes at 40 mph, and 20 minutes at 20mph, with two 3 minute stretches at top speed in the middle. The bike feels perfectly fine for the first 10 or 20 minutes at 40 or 20mph, then I go top speed and right on cue after 30s, at the same place every time, the power flicks down just a notch, and returns, and down again, etc. At the end of my 3 minutes at top speed, the power loss is permanent for the rest of the trip. It does not help to ride at 30 mph for a few minutes and then try top speed again. Once the power is gone, it's gone. When I ride again 7 hours later the bike is fine, until I do top speed for 30 seconds again.
To investigate I once stopped the bike for 5 minutes, then tried to reach top speed again. I got nearly to 55mph but the pulsing started immediately and in half a minute I was back down to 45 mph. This all implies it is temperature related to me.
It seems to make little difference, if any, if the day is icy cold or mild, though.
The garage says that they could not have caused the problem, but that if I bring the bike in they will check the [something] - little rings that change gear and might not be quite round anymore so may be sticking. But they will charge me for the work if that isn't the problem. But how can it be caused by that when the bike runs perfectly fine up to top speed for 30 seconds?
Someone else suggested it might be water in the carburettor, but then surely there'd be regular interruptions to power at any speed.
And final suggestion I have so far is that it might be a dirty filter (or something), and the engine is getting starved for fuel at top speeds. But then it should be back to normal after slowing down for a while.
Please help if you have any ideas!
Thank you.

[Andy_Pagin]

How many miles has it done?
What did they do on the service?
If they changed the drive belt they may well have damaged something, or fitted the wrong belt, or it could be coincidence and the belt is on the way out.
Spark plug - did they fit the correct one? Is it torqued up properly?
Try pulling and re-fitting the plug lead, could be as simple as a bad connection.
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[antandcharmi]

Thanks.
5,000 miles.
Don't know what they did on the service but they mumbled about oil and spark plugs.
Bike runs perfectly with full power even at top speed until 30s have gone by at top speed only, so can't be anything that doesn't vary e.g. with fuel usage/temp/speed.
Just had a thought this morning though. It was very cold and the bike on pulling away behaved very similarly to the top speed problem, switching max power on and off, until it warmed up. It has a cheap Chinese-made choke with some kind of wax stiffen/soften by temp controller apparently. Now at operating temp and full speed the choke won't be the same but maybe it is still interfering somehow?

[chris-red]

Next time it does it stop and take the fuel cap off, if you hear a sucking noise like air rushing in it could be a kinked/blocked breather hose. Other wise I'd be checking the fuel system, petrol tap, hoses etc.
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[stevo as b4]

Id check the drive belt and the variator rollers first, if you have no reason to doubt that the engine is running properly. Flat spotted rollers or wear marks in the pulley halves could cause the kind of symptoms you are seeing.

[antandcharmi]

Thanks for all the replies. I would think if it was fuel though, that it wouldn't kick in and stay in for the rest of the trip?
Re the variator rollers, surely they wouldn't go flat in a service?
BTW I said 5,000 miles but it's km! I keep thinking in miles.
I think what I'll do, not being mechanically minded, and the workshop saying it's not them and can't think what it can be, is take it in to another (more expensive) workshop and ask them if they can cure it.
But thanks very much for all the time taken, guys, I really appreciate it. I feel less alone with this mystery, at least.
When/If I get it cured, I will update you all FYI.