Heater blowing cold air

[RThefarm@domain.elided]

Can anyone tell me what I can do about my heater?  Sometimes it will blow 
warm air for a little bit, but then starts blowing cold air.  On at least one 
occasion it has switched back to blowing warm air for a little bit.  Very 
currious!  Im thinking that there must be either a bubble in the cooling 
system, or maybe there is a valve somewhere that is shutting off the supply 
of hot water to the heater...  Well thanks in advance for any help.  

Chris Rundlett
'82 Spider Veloce
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