Spi Will Not Run

[MAD400EX]

I have spi 580 95 it runs all day one day, The next day it runs for 20 minutes then it will not run full throttle , 1/2 throttle it idles ok if you give it gas it will not go and if I stop for 20 minutes it will run for 5 minutes.
I Replaced the Spark plugs. Thanks