Shifting Problems

As I was driving, the speedometer drop to 0 and the RPM's jumped up to 4000. I took my foot off the gas and tapped on the gas again. I got some accelration, but it did it again. What is causing this? Do you Know?

I guess a crazy camshaft/crankshaft position sensor could do something like that.
What make and year vehicle?

wow, 4000 rpm? did you actualy hear the engine roar to that or was it a faulty reading? and what driving conditions? starting from a stop? accelerating in traffic? just cruising? chris is right, lot more info needed here.