most thermo's open at 70-85c (i have the qh thermo book which lists them all
, average is around 72/3c )
and yes its a common as muck vw golf mk4 fault with the thermos opening to early, they jsut go sloppy and dont shut fully, shove a new one in and all is done. also the coolant temp sensor can start to fail which can cause the needle to read low (will either be a 2 pin green or 3 pin blue plug on the top rad hose) jsut change them both when the system is empty of antifreeze
had your windows fallen out yet? got to love that fault to! (most reliable car i have ever owned in my life, not lol)