2007 Mazda B3000 hard to shift (manual) when hot

I have a 2007 Mazda B3000 Dual Sport (2 wd)purchased in December 2007 new. I traded in a 2004 truck of the same brand and model to get it, so am familiar with the driving characteristics. It seemed to run great as a new car should. However, after a few thousand miles, I decided to take a trip across the island to Kona (I live in Hilo, on the Big Island of Hawaii). Prior to this trip, I noticed that on occasion, it seemed like the transmission would be rough…there seemed to be some tension in the system when I’d take it out of gear, and it would sometimes be hard to get INTO the next gear, especially 1st and reverse. During this trip, which has many hills and valleys en-route, the symptoms, after about 45 minutes drive, from 0 to 4000 ft elevation change, became worse. Even while driving, it would be hard to shift…it just did not want to go INTO gear, and sometimes would grind. Pumping the clutch didnt seem to help. After several trips to the dealer, (they suspected air in the hydrolics), they finally changed out a part in the hydrolics. This part (I forget the name) is shared with the brake system according to the mechanic. The problem was fixed and even on long trips it shifted very smoothly and easily. At this point, the truck had about 7500 miles on it.

Now I have about 17000 miles on it and on long trips to Kona or up to Volcano (national park, at 4000 ft), it begins to show the same symptoms, though this time, when I pump the clutch, it seems to help and there are no ‘scary’ times when I just cant get it in gear like previously. On short trips into town, (about 20 miles, no hills), it is fine, but on the long trips, it becomes an issue again. They can not duplicate the problem at the dealer as nobody is willing to drive up the mountain for an hour to see it happen, and they indicate that the part replaced is brand new and there is not much they can do about it.

Any recommendations?

I would check the oil level in the transmission first.