1998 Dodge Neon overheating and speedometer

I bought my car about 3 months ago and over the last month now ive had to keep filling the radiator fluid up periodically. Its not leaking anywhere, the car overheats at times and when I shut it off it boils into the resevoir. Also the speedometer bounces and stops at times. I have read around about possibly the speedometer is causing the overheating problem cause it doesn't allow to kick the fans on but im not sure and I would just like to know some suggestions before I pour money into something that won't make a difference in helping my car.