peugeot jetforce?

Hi! i've got a peugeot jetforce (125 abs efi) only had the bike 4 weeks it's a 2004 model only 3500 miles on the clock, the person i bought it off said it has been left standing for 5 months. so i changed the spark plug and engine oil, it's been running fine for 3weeks 65mph no problem, suddenly the bike is losing power, it struggles to do 40mph! is there any one who could help me with this problem? i would be very gratefull Cheers!