I have a 2009 Mazda 5. Great car in general with few problems.

Lately the manual transmission clutch has been acting weird. Sometimes, after the car sits for a while it will not go into gear. I press the clutch and it either will not move, or sometimes will sort of bounce two or three times. A week ago I was at work, took the car out at lunch time with no problems. 4:30 it would not go into gear. I was able to sort of match RPMs to get it in, but it was a perilous journey home, luckily mostly highway driving where i could stay in 4th gear. But, once I got into town and had to stop at an intersection on a hill it was a nightmare: would not go into gear, stalling out when I could jam it in. Finally limped home a mile in 1st gear.
Towed it to good mechanic who could not replicate the issue. Replaced the slave cylinder, and it ran well for a week. Yesterday tried to drive, again after car sitting for a while, and had the bouncing issue again and could not get it in gear. Tried pumping it, but it would not even go to the floor. I had to borrow my wife's car for urgent errand. Called her later and she went out and tried it, no problem. Ditto trying it hours later when I returned. Today, drove to work with no problems.

But I'm afraid it will act up again. My area is hilly and I don't know what I will do if I am stuck at an uphill intersection.

Any ideas what is happening?

Thanks in advance for help and advice.