When I am driving my truck it will not go over 40 mph sometimes it revs the **** out of it and makes a loud knocking under the hood sometimes after it does that for 10 minutes it works fine sometimes it dont and sometimes i dont have a problem with it at all but its a real bitch pulling out on to main roads where wont get up to speed cant figure out whats wrong with it any ideas

 

I have seen a bad O2 sensor cause something like this. But did you get the codes scanned?

 

A plugged up catilytic convertor will do the same thing. Does it blow any black smoke? A fuel delivery problem will also cause this.....As said...scan the computer for codes this will help ALOT.