RPM's Jumping grand prix

[bengalnut]

I've got a 98 Grand Prix
When driving at 60 mph, my RPM's jump from about 1800 to about 2300 or so then back down to 1800. Sometimes takes 15 minutes to start happening, sometimes happens shortly after starting. Also happens at slower speeds, but is not as noticable. Does not "jump" much in idle. When driving it is very regular in timing usually about every 3 seconds, but sometimes every 4 seconds, sometimes every 6 seconds. Any info would be greatly appreciated.