Hello I am new to this forum and just recently acquired a 06 v70. The temp gauge jumps up to halfway within 1 min of driving, and fluctuates slightly. Is this normal, car had a heater hose blow and had a new heater installed at that time. I was thinking that it might be a head gasket issue but really unsure. Any help would be greatly appreciated Thank you new v70 owner.

 

How many miles are on this 2006?
Did this car come with any service records?
Did you buy it "blind" off a lot thus taking the word of the salesman or was it a private sale?
Do you work on cars yourself?

I've read the temp. gauge takes it's signal from the CTS (Coolant Temperature Sensor) then electronics at the DIM (Driver's Information Module) process it into a needle position. It's not a direct read gauge.
My train of thought on your situation is this. Given that the often faulty DIMs were updated/corrected years ago, yours is likely OK. That leaves the sensor as the possible culprit. Throwing parts at a problem without knowing EVERYTHING for sure is said to be foolish. However, those sensors do go bad and they're not expensive.
Locate yours and decide if you want to do it. Order a CTS online and install it. They're smaller than a spark plug. You disconnect the wire and unscrew it.
It's real simple to change.

Regarding the heater hose and heater core(?) and possible head gasket failure.
How long ago was this done? Was it addressed by you or the PO?
Opening the coolant reservoir and smelling for exhaust is one time honored to check for HG leakage.

 

Our cooling systems are forgiving. Just fill the cooling system as completely as you can when you're finished.

Fill until the radiator is brimmed, squeezing the top radiator hose for the few bubbles you'll get. Close the cap.
Then fill the reservoir 3/4. Close it's cap and run the engine to warm. When the engine cools it'll suck coolant in.
Keep the coolant between the "MIN" and "MAX" marks.