Help Please! Clutch in, Idles to 4000 RPM

I own a 1994 Mazda B3000 V6. It has around 100,000 miles on it. My issue only happens ever so often, but lately it has gotten worse. When I drive the truck, I have to shift at a very High RPM, around 3500 RPM, just to get this problem to stop. Of course it does not always stop. What is happening is that when I go to shift and push down on the Clutch, the RPMs go racing to 4000 plus and after I shift they go back down but then start to climb again, without my foot on the gas. I could be in second gear and the truck will accelerate gradually by itself, without any gas pedal being pushed. When I come to a stop sign and have the clutch in, the RPM's will stay at 4000's for around 30 seconds or even more. When they go back down and I go to shift again, they go racing back up.

Any help is appreciated! Thanks.