Car overheats

[Jstudnicki]

Hello I have a 214 c300 merc benz.
It seems to be heating up the temp jump between 100 and 120 c.
The heater seems to work sometimes when the car is at low temp.
When it starts acting up the fan kicks into high speed and stays on and the heater doesn't always work.
When it gets hot there is alot of pressure in the top radiator hose. When it's cooler there doesn't seem to be much pressure in the hose.
And when u hold tge hose tight u really can't feel water gushing thru the hose.
The temp seems to jump around between 100 and 120 c but like I said the fan stays on n it seems to cool off.

What would be a good starting point for a repair?
New thermostat n housing ?