How do I fix sudden RPM surges in a 95 Mustang GT 5.0 SN95?

How do I fix sudden RPM surges in a 95 Mustang GT 5.0 SN95? After driving 15-20 minutes the RPM gauge jumps from 1 1/2 to 2 RPM and the motor rev's up like I'm pushing down on the gas pedal but I'm not. My foot is steady on the pedal. I think my engine fan is turning on when the RPM gauge surges and turning off when it stops. Why is it doing this and how do I fix it? How do I know if it is the engine fan or not?