1998 Ford Ranger shifting problems

After about 5 minutes of driving my transmission will not shift without a lot of effort. From a stop or while moving, first and second are almost impossible. I noticed too that it seems to be idling higher when the problem exist. I'm not sure how it relates. (Note that when the RPMs go up the air blows warmer; however it only jumps to around 1000 to 1100 from around 700-I realize that this all could be unrelated, but they are happening at the exact times.)

When I start driving it, I have plenty of clutch pressure, but when the problem starts, I have almost none. If I kill the engine and then shift to first, when I start it back up it starts rolling as the clutch is not engaging fully. The clutch is not slipping at all.

I took it to the shop today and they were sure it was the master. They changed it and made sure that there was no air and the problem didn't go away and may be worse. They also stated that the slave is fine as well. They now want to drop the tranny to look around but have no idea what it is. That said, I'm at $850 out of pocket with no solution in sight.

note: The current clutch and slave have 30,000 miles on them and have been flawless up until the last couple of days. All of the miles are highway miles and the truck is in excellent condition with absolutely no abuse.

What is strange to me is that when it works, it works great with plenty of clutch pressure. I don't get how it's fine one moment and not the next.

Sorry about all of the details, but I wanted to share all of the details that seem to be surrounding the problem.