1995 yamaha xt 350 cutting out
 

Hey guys just bought a 1995 yamaha xt 350 runs good ut cuts out when driving. Idels fine , revs up fine in neatral but as soon as i drive it it cuts out at the 4000 5000 rpm range then surges and goes good again. I changed plug cleaned carb with no difference. Can anybody help also i noticed the engine has a small knock like sound is this normal for these engines thanks guys