You could try completely removing and reinstalling the drivers. If even that doesn't fix it then it's probably a hardware problem.

The main reason for the change in fan speed is most likely that the card is hovering around a temperature threshold. Most cards have a threshold based fan while a few high end cards will allow for flashing a new fan profile to bios (e.g. Asus ROG cards would let you set a custom fan profile into the card bios).

E.g. the thresholds are at 55, 60 and 65 degrees. Fanspeed at range 55-60 is 2300, 60-65 is 2600. The card is hovering around 60 degrees and when it spins up to 2600RPM the temp immediately goes to 59 and 2300RPM, but 2 seconds later the card is back up to 60 degrees and another 2 seconds later it is back down to 59 degrees. This would cause a loop with fan speeds going up and down constantly.

Is the card old? If so, try cleaning the cooler. Vacumcleaner can work, but make sure the nozzle doesn't touch any computer components as the static electricity can kill your card. Also make sure that you hold the fans so they do not spin when vacuuming. Alternately you can dismount the cooler and remove fans and shroud to clean it with a brush or similar. Dust between the fins on the cooler can cause bad cooling and severely reduce efficiency at low fan speeds.