what could be the cause of heat blowing hot and cold air off and on? 99 buick pk ave ultra. just had radiator flushed and filled?

I am not sure exactly what you are describing  [i] heat blowing hot and cold air off[/i]

That said, if a heating cooling problem developed right after radiator flush and refill, I would suspect air in the system and whoever did the work likely will correct the problem for you.  My next guess would be a vacuum hose that might have been accidentally disconnected during the work. 

Could you tell us the mileage on the car and a more detailed description of the problem?