I own a 1974 TS250, when its running and driving it seems like its missing a bit. It seems to be fine when giving it gas or cruising in 5th. Any ideas what could be wrong?

if you only feel it when you are motivating down the highway then it could be your clutch slipping. Try draining your oil flushing the system and adding properly rated motorcycle oil not car oil that you buy in the parts store or wally world