Bmw 523i se heater blows hot and cold

SOURCE: BMW 325xi 04 Heater Fan working (sound) but not blowing any air.

motor is going bad replace it

SOURCE: over heating prob on 98 bmw 523i se

the problem could also be your radiator cap and you need to make sure that you don,t have an air bubble in the system you also have to have the right amount of anti freeze in it but try the cap first seems so simple but it is a good possibility. hope this helps

SOURCE: cigarette lighter not working - which fuse and

usually it is with interior light/gauge lights. or it will say on the cover "cig lgtr"

SOURCE: the engine seems to overheat and the internal heater blows cold

The problem is your coolant level has gone too low check and top up when engine is cold, you will see the marks on the expansion bottle, if prob persists there may be an airlock in the heater radiator. keep an eye on coolant level or you will have cylinder head problems.

SOURCE: i own a bmw 523i 1998, when stop at traffic light,

Hi There,
Hopefully i can point you in the right direction mayeb even offer a solution,
Generally i have foudn that when this happens its mainly due to a vacuum leak somewhere in the system. Check all the small rubber hoses in and aroudn the intake manifold.
If they are all non perrished and in good condtion it coudl be something else such as a clogged injector, However there are soo many possibilities and problems that it could be.
With cars the main problem is that sometimes its a few different things causing the one problem.
Has the car been tuned recently/Serviced.
Generally if air filters /fuel filters are blocked and dirty they can also help cause hunting in engines.
It could also be that you throttle linkage is sticking. This is common in older cars as dirt grease and oil builds up on the engine and can stop linkages moving correctly, 
You coudl try following the wire that runs to the accelerator/gas pedal to the butterfly and try cleaning and lubricating with a reputable spray on lubricant that wont attract dust on all the linkages carefully and see if this helps .
Other common problems may be timing chains/belts, If they need replacing they tend to cause spiraddic idle as well although generally it would be really bad all the time,
Sometimes however it can be a problem with the cars onboard computer, be aware that if your cars battery is running low on charge or is long past needing to be replaced this to can effect the computer and cause spiradic idle as the car trys to compensate revs to push more charge through the alternator.
I know its not really a definitive solution but i hope it helps save you takinng it to the mechanic.
Cheers
Steve