2005 CE Manual Clutch Problem

I have a 2005 5-speed manual CE with 140000 miles on it. The clutch has been slipping, intermittenly and only slightly, for three years. It never slips in 1st or second, and until the last month not in 3rd, but mostly in 4th and 5th. This past summer was hot and dry and it was ok, but with a wet Fall it's acting up again. It seems to slip more when it has been raining, and when wet it also sometimes shudders when starting up from a stop. The slipping is very brief ( 1 second or less) as I immediately let up on the accelerator and go easier on it when resuming. When dry I haul an ATV on a utility trailer, even up hills but of course not in 4th or 5th, and it is fine.

The first time it happened, in 2008, the Toyota dealer said it needed a new clutch, without ever looking at or driving it. I decided to wait a bit as it only happened a couple of times in a month or so, and here 3 years later it is more or less the same. I have asked 3 or 4 mechanics their opinions and they have all acted baffled. They tell me that if they have to check it they might as well replace it because of the amount of work required to get at it.

It acts almost like fluid is getting on the plate from somewhere, possibly rainwater. I've driven manual transmissions since the fifties and have never seen anything like this. Does anyone have any similar experience and/or any ideas of possible fixes?
It is a tribute to Toyota that a clutch would last 140,000 miles. I got my first clutch car, a 54 Ford in 1959. The clutch chattered on start up, so after some time I replaced it. (55,000 miles) It had oil on it. Clutches slip in 4 and 5 because the "load" on the clutch is higher, due to aerodynamic drag and rolling friction. The slipping may also heat and warp the flywheel. Dealers are always very expensive, find a local mechanic, get a new clutch, flywheel and hydraulics. I just put a new transmission, torque converter in my 2002 Saturn Vue myself. It is a lot of work to do that, worth every dollar.
I have had the same type of slippage for about 2 years now. My 06 5 speed has 135,000 on it.

Has anyone replaced their own Corolla clutch?
My stock clutch lasted clear up to 282,000 in my 04 5-speed and it wasnt even bad when I pulled the transmission. I just figured id put a new one in when i put a new transmission in.