Transmission is not kickin in with the gears what could be the problem?

I have a 98 Ford Windstar and when I drive it sometimes the RPM's go way high and dont come down other times they dont move at all. I want to know if you guys could give me any pointers as to what it could be. Is it the transmission? or is it the motor? What could be the probably causes for which the RPM's are acting funny. The symptoms of the transmission include surges, loss of velocity at times, and the troubled codes the obd reader keeps givin me are P0174 any help is greatly appreciated thank you.