Toyota Repair: Clutch, clutch master, clutch pedal

Question
I'm driving a 91 240sx Nissan, yes it's not a Toyota but I guess this is more of a general question.

The clutch is temperture sensitivity.  When the weather is hot or after a long highway drive, the release point is much higher.  A few ocassions the clutch slipped so badly that the car couldn't even move.  I've found the clutch pedal is very light when it's hot and hard when it's cold ( @ night).  Clutch was replaced 1&half year ago and I'm driving quite gentle.  Clutch master and slave cyclinder had been replaced.  
Do you have any idea of what the problem is?

Your help is much appreciated.

Answer
The only thing I can think of at this time is that the fluid may be contaminated, it is possible that oil or possible another petroleum based fluid was added by mistake, I recommend flushing all the old fluid out and putting in fresh brake fluid and see if that helps.