it is a Yamaha Zeal 250, 1999 56,000km. on a long uphill if i am going in gear 5, speed is 60-70, reving at 4000, bike will start loosing power before i reach the hill top. the rev stays at 4000, throttle response is very strange, it seems to give a little bit of power but not continuous and consistance. feels like as if i am riding on a buppy road. if i shift to lower gear engine will immediately goes off. so i will grip my throttle to get myself to hill top (if lucky), at very low speed around 20 before it stall itself. if i come to a uphill in gear 3 this problem does not happen but on a main road with a speed limit of 70 or 80, and it would be very strange to shift to gear 3 and slow down considerably before i start claiming. the bike has just been serviced, spark plug, oil, and oil filter changed. any thoughts? thanks a lot!