My 1995 SW1 has a bad over heating problem, especially when sitting at idle, or in stop & go traffic, the temp builds and never starts to cool. I am not sure where to start to diagnose the problem.
The electric radiator fan. start there.
Check to see that is is turning when the car gets hot. Check the relay - swap it for an identical one in the fuse block, Fan run? - replace relay. Check to see the relay is getting a power and ground across the signal terminals. No ground (the ECU likely pulls the relay signal to ground to command the fan) when the fan should be running? Probably the coolant temp sensor.
Check to see the fan is getting power. If it is, jump it with battery power - if it turns, the fan is OK, if not replace the fan.