Mechanical question about my Kia?

I have a 2000 Kia Sephia, every now and again the speed-odometer will fail and will go up and down continuously and my over drive light will flash on and off. Does anyone know why and how to fix it? Also another question I have is every time I put gas in my car, it sputters and stops, as long as I keep pushing on the gas it will eventually go on and have no further problems until my next fill up, can anyone tell me what I can do here? 10 points will only be given to the person who doesn't say things like "look it up online or read a book", I want an actual knowledgeable person answering this question, if u don't know the answer then please don't answer!