Upstairs there is a problem, but not downstairs. That suggests a pooling of warm moist air at the top of your house.
Is the downstairs open plan, or can doors downstairs be shut during the day and at night? What happens if you keep the doors shut if you can?
Do you shut rooms upstairs during daytime and at night? What happens if you keep the doors shut?
Does "upstairs" refer to slept-in bedrooms or all rooms (e.g. any unoccupied rooms) or windows upstairs e.g. on the landing?
Do you have heating on hot during the evening, but off all night? Can you thermostat your system to keep low level heating at all times, except when you need to increase it during very cold weather?
Do you have thermostatted radiator valves?
Can you reduce the temperature of the heating upstairs using TRV or manually restricting the valves?
Can you keep a desk fan (i.e not heated) at low speed pointing at the worst window for 24 hours, and see what the difference, if any, would be?
Can you ventilate occupied bedrooms for a short period 15-30 min in the mornings by opening windows wide with the door shut, on cold or dry, but not wet or damp weather days? (on very cold days the outside humidity will always be very low, but on mild days the humidity can be high)
To sum up - compartmentalize your house to prevent a convected temperature gradient; heat less but continuously, not intermittently high; keep bedrooms cooler; increase air movement over troublesome condensation areas to prevent those areas from being colder than the rest of the room; ventilate problem rooms when conditions allow.