Engine is revving but not going when in gear

The RPM's will suddenly go up to a very high number (without the car accelerating), then it won't go over 15 mph. It doesn't matter how fast you are going all of a sudden the rpm's will go up to a very high number and the car almost comes to a complete stall and "putts". After letting it sit for a little bit, I can restart it and it will be fine for a little while and then it will do it again. I had one mechanic tell me it was the transmission and had the transmission filter and fluid replaced (before spending big bucks on a new transmission) another mechanic said it was the spark plugs so I had those replaced. I have had another mechanic tell me that it sounded to him like a converter solenoid. Any ideas why my car would be doing this? Thank you!