I assume you mean "When your hot water needle does NOT work." Thermostat.
could it be the thermostat or the fan itself?
As engine has to work hard and engine RPM are high its temperature rises and water pump also works faster and hot water reaches temperature sensor fast and suddenly needle goes up.
Could be a vacuum leak.
90- degree
No, the needle is the same as any piercing.
No. Too hot is when the needle gets about 3/4 over on typical guages, sometimes it's marked in red, sometimes not. Either way, there should be a marked line around the 3/4 position on the guage, past that is not good.
Multiple problems could contribute but I would need more details such as does it turn off on it's own or when you turn it off the needle jumps then the car stops running.
When a car is moving with constant velocity, the speedometer needle will stay steady at a particular speed. When the car is accelerating, the needle will move clockwise indicating an increase in speed. When the car is decelerating, the needle will move counterclockwise indicating a decrease in speed.
If the speedometer needle is going crazy and not working properly, it is probably the speedometer mechanism that needs to be replaced. It could also mean the vehicle is having a transmission slippage problem if the needle is bouncing back and forth.
The machine that has a needle that goes from side to side is an overlock serger. It is used to prevent fabric from fraying.
You could have a bad thermostat, a blown head gasket, a cracked head.