Temp gauge goes down???

[mattbarber]

Hi all,

54 plate 1.9 AWX 147k miles.

My mums Golf Mk 4 1.9 TDi 130 does the same.....in higher RPM in 2nd or 3rd gear...say around 4-4.5k RPM for a few hundred yards, I've seen the temp gauge drop a bit. Then on more normal RPM it creeps back up to 90 degrees and stays there.

What would cause that? Not head gasket is it?
You'd think that higher RPM it would stay at 90, not go down?!