Hey Guys... just jumping here because it sounds like you know what youre talking about and can possibly help this total rookie. I have a 1978 Motobecane VLX. I have had it for a few months.. When I start it and start riding it, it takes off and has good power. But after about 5 minutes or about a mile or so, it starts to get weaker. And starting from a stop is very slow and kind of bogged out, versus when I first fire it up. What do you think is happening here and do you all have any suggestions?