Lets go with the simplest most possible reasons;
1. Is your car stock? Or modded? What kind of mods?
2. Does this happen when driving highway and when in moderate to heavy traffic? It is normal for your needle to jump up a bit when you are in traffic and to go down when you're on the highway free of traffic. Though 70 to 100 is extremely cold and you're running extremely rich.
3. Have you tried bleeding out the air of the system?
4. What kind of thermostat do you have? If you don't know, you should probably change that out first.
5. Is your coolant level correct? Check if it's dirty with mud like crap on the radiator cap. Is your radiator full of just water or just coolant and not 50/50 mixed? (make sure the car is cool of course before doing this)
6. Other than that, are the fans kicking in at the right time and are they both working?
EDITED TO INCLUDE Bio248s suggestion.
