Temperature indicator B4 2,0l Motor ABK

Hello dear Audi Whisperer, with my B4 2.0l, engine: ABK has been extremely sluggish for some time now: It doesn’t move at all for a long time and then – after a long start – goes so to 60°, max 80°. Have changed the thermostat, but everything is the same. What is it? What to do? Replace the indicator instrument or is the thermosensor defective?