My Geo Prizm has 307,780 miles. The motor started to hesitate going 60 mph

Thanks all the same, I appreciate any and all help! My youngest son, also mentioned something about pistons and rings, too! But, the car only makes the noise when I go, 60 mph on the highway and not in the city at 45 or 30 mph. And it stopped after I changed the plugs. Now, the gage on the dashboard that tells you how hot or cold your car is running, is acting up. What does that mean? If I drive my car in the city only. Do you think,the car can last a few more months till I'm able to find another replacement.

SOME TIMES ALOT OF CARBON BUILD UP ON THE EXHAUST VALVES CAN SOUND LIKE A ROD KNOCK, A PICE CAN BREAK OFF AND HANG THE VALVE(S) OPEN AND THE PISTON SMACKS THEM BECAUSE THEY ARE NOT CLOSEING ALL THE WAY, THEY MAKE A PRODUCT THAT CLEANS THEM OFF AND GETS RIDE OF THAT, CONTACT YOU LOCAL AUTO PARTS STORE AND ASK THEM WHAT THEY HAVE FOR TAKING CARBON BUILD UP OFF OF ENGINE VALVES IN THE CYLINDER HEAD

Really! I found out, there are 3 other people here in Gulfport area who also has a similar make & model red car. And I found most of the parts I needed for my car at O'Reilly's, Auto Zone, fairly reasonable. And for older parts from Pull o Parts, by the Menge Fleamarket, behind the Dollar General store. My son gave me this car back in Texas, then it only had, 98,000 miles on it, back in 2008. He called it a Frankenstein car! Because it has parts from, Datsun, General motors, Ford, Chevy, Nissan, and TOYOTA.