what are the reasons for the cooling system to boil or fluctuating cooling temp on 1998 BMW 540i

also heater is not working properly

2 answers
Thermostat problem, most likely.
Sounds like the thermostat is bad. Need to replace soon but make sure coolant is at proper level and not leaking. Other cause is heater core obstruction not allowing fluid to flow through properly. Hopefully, not the water pump.