When I drove onto the off ramp (no traffic jam there) the needle went down
rapidly to below what I am used to.

I checked oil and water several times still no drop in level so there does
not seem to be a problem there (I did have a leaky hose earlier this year)
and managed to drive home without any incidents.

However I did notice that when driving at speeds above 25 mph the needle
stays right in the middle, but  when I slow down it tends to go up. When
increasing speed it goes below of what I am used to.

In my laymens opinion this indicates a thermostat problem.

I'm a laymen myself, and this indicates a cooling fan problem.  No problem
when your moving right?  Your not getting airflow when in traffic.