After driving about 100 miles I noticed the temp gauge slowly rising to the 3/4 mark. when I pulled off the freeway it went back to normal. I continued to drive another 30 miles or so and the gauge started to rise again, and finally pegged the red mark. I pulled off the freeway again to get to a gas station where I could check it out. Before I got to the station the gauge went back to normal. There was no sign of water leakage, but decided to stay the night at a hotel and have it towed o a mechanic the next morning. The mechanic performed 2 pressure checks and found no problem. They replaced the radiator cap and charged me $130.00. On the way home the gauge went up to the 3/4 mark so I pulled over. when I was fully stopped the gauge went back to normal. I decided I would continue to drive home (another 30 miles) and did not have any problems. Now I`m not sure if I should bring it in or wait it out. Any ideas on what it could be????
Thanks [Edit]

 

Can I know your Car Model And Make.

 

I'm sorry, It is a 2001 300M.

 

Do you have an electronic cooling fans on this one
If yes I would check if the auxullary is working or not
It will not coast you alot to replace it also to be more sure change the thermostat and thats it hope it will run fine else if it still happens then you need to see if there is any trace of coolant that splash on the front of the engine and small amount of coolant is missing every week which means even you have a radiator damage in the medal of it or waterpump leak