You pretty much figured out the problem or the most likely problem when you created this post. "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."
1.Heater doesn't work because there isn't any flow of coolant.
2.There is no pressure when cooler because engine isn't at temp.
3.There is a lot of built up pressure when hot in top hose because the temp, chances are the lower radiator hose isn't hot at all.
4. You can't feel the water flowing through the hose because the pump either isn't working or you have a major blockage.

Why don't you either test the water pump or take it to a mechanic. An overheating engine isn't something you want to mess around with if you aren't really sure of what's going on. A $200 -$400 cooling problem can quickly become a several thousand dollar head gasket job or complete engine rebuild.

__________________

1990 190E 3.0L