Engine starts great. Idles great. When given full throttle it takes off great but in a few minutes it starts to slow down. If I keep the rpm's around 3500 it runs great. I can idle it around a bit and give it full throttle and it takes off again but soon it slows down again. If I back it down to around 3500rpm's it again runs smooth as silk. Could it be the fuel pump? I have replaced the fuel filter and spark plugs.