Q: Having problems with clutch

Over the last few weeks, I have found that when the car is warmed up, I have trouble shifting between gears. But, this is an intermittent problem. While the car is in motion, shifting gears is smooth; however, trying to put the car into gear from a standstill gets difficult. There is no slippage, no whining, grinding or clunking. I have replaced the transmission fluid. The clutch fluid level is okay. There are no leaks that I have found either. I finally took it in and the mechanic, simply by test driving it, told me that I need a new clutch pack. I also visited transmission shops, as well as a number of independent auto mechanics, and they tell me the problem is of a hydraulic nature. Do I need a new clutch pack? Or is it something else?