cooling problem

[Bernard Laniel <blaniel@videotron.ca>]

I own a 1990 audi 90 with the 10v engine and I'm having trouble with the
thermostat that is sticking close.  2 months ago the car was
overheating, I changed the thermostat and everything has been fine since
but today the same problem is happening.  What could be the reason for
the thermostat to stay closed like that and what is the normal operating
temperature for these engines, mine is showing around 70 to 75 degrees
celcius.