1998 Other Mitsubishi Models Can you help me in how I might

I have a 1998 Mitsubishi Colt that for the last week has lacked alot of power and also judders all the time. When at high RPM it judders less and judders very badly when lower or at normal RPM. It also seems to be using alot of fuel. Even though I am know where near a car expert I thought it may be a fault with my O2 sensor but I can't be sure at all. I also can't afford to get it fixed yet. Can you help me with a possible diagnosis and give me some advice in what to do with the problem. Is it something I caould do myself with a friend who is an ametuer machanic or try myself with a manual you recommend. My anitial thought is that it isn't to serious becasue I can still do a steady 70 MPH but it takes an age to get there. If it is possibly something simple could you please advice me where to look so I can be sure of the problem and then I can try to look to get it fixed. Thanks alot.