1994 Honda Accord fluctuating clutch pressure / slip

2 months ago I had a clutch master cylinder pump replaced. Before the problem all worked fine, car ran like new. Now the clutch pressure fluctuates. In morning with cooler weather it is very soft, but after driving about 10 minutes or so clutch will be very stiff and transmission seems to slip somewhat especially on heavy traffic or stop and go due to stop lights. Will also seem to be lacking power when taking off-could need a tune up or new spark plugs? Shop who did work sez there is another hydraulic punp that may need to be changed but am very skeptical of there work or word thus far. Any suggestions? Please help!