There is an issue like this on Corvettes too, I just have it happen on my car where the pedal stuck and I had to pull it up with my foot and I only got 18,000 miles on the car.
What we do to fix the issues is to remove all fluid, bleed and repeat till it is clean fluid in the reservoir.
Try this and if it doesnt work I would replace the slave cylinder and clutch master cylinder because 1 or the other is wore out and by replacing just 1 part will probably make the other part fail soon, seen it happen way to many times.
Though I know you dont have a Corvette but this link can show you why bad fluid does funny things. Hope this helps a little.
Clutch_Care
