TDI 300 gauge reading hot. cools when you drive hard

So my tdi 300 started running hot (just before the red mark). When i put the foot down and kept the revs up she ran cooler but not at mid range.

No coolant leaks although the tank was low so I topped it up. No steam etc.

I removed the thermostat and ran her - still the same temperatures readings on the gauge.

When I played around with the temperature sender it would occasionally drop back to the normal mid range on the gauge.

I hear the standard temperature gauges are pathetic to say the least.

Any suggestions folks?

I know! Resistance on a sender shouldn't change like that by twisting the plug around! Its just the fear of killing the engine that brought me to ask on the forum to see what others think! I am currently waiting for her to cool completely to see what happens to the gauge when I fire her up from cold.