2002 Nissan Maxima overheating hydraulic clutch

I was wanting to know what might be the problem with my car. About a year ago, I was traveling back from TJ with my church group over the border crossing. It was hot, and of course the traffic was long. Of course, you also have to shift constantly to move in traffic. Well, the clutch pedal seemed to lose pressure; the pedal went straight to floor, and I couldn't shift into any gear. After I crossed the border, I pulled over for about 10 min. and it seemed fine. When I got back, I took it to the dealer, who said that since it wasn't having the problem when I brought it in, they couldn't fix it because they saw nothing wrong when they did their inspection. Since then, no more issues.
However, last weekend, same scenario. Hot temp. and slow traffic, so my clutch failed yet again. This time, I had to be pushed about 1/4 mile over the border, where I had to wait over an hour and a half. This time, it didn't get better, and eventually I had to have my car towed over 150 miles back to my church. When we got there and the guy took it off the towbed, it worked fine. Again, the dealer is telling me that unless it is doing what I described when I bring it in, there's not much they can do to help me, and my warranty is about to expire soon.
Now the fluid is dark, and smells burnt, and I just recently had a 'major service' at Nissan where they were supposed to inspect/change all fluids where necessary. I can't imagine it burnt like it did in one episode at the border. I keep insisting that something's WRONG w/the car, but they blow me off like I don't know what I'm talking about.
Someone told me it could be my slave cylinder, or that I need something installed to cool the clutch fluid. Another person thought I might be somehow getting air in the lines. If so, how could that be if the line is sealed?
Any advice you can offer would be appreciated!