No Heat -- Tried everything except????

I am by now well aware that this Dexcool thing is bad news. I have had 2 mechanchs attempt different things and one is a retired GM mechanic who worked in the plant I believe. Both of them said Dexcool is a trainwreck and I now have extended life but what damage is done I cant say. I have no heat, by my engine runs cool. We have did pressure flushes 3 x as of today, replaced the thermostat, the radiator, the water pump, and the system is clean as a bell and has extended life. There are no plugs as far as we can see. The mechanic said he has seen before where there is an air-pocket (which is indicative of a bad seal in a head gasket or elsewhere) but that was negative. The only other thing they can think of is perhaps a little door that opens and shuts some place that acts like agate valve perhaps? to get at it would mean to take the dashboard out etc and now its getting more expensive than perhaps what the car is worth. But I love this car and everything works awesome and I have put tons of new parts into it to keep it going in all areas of the vehicle. I hate to lose it and it has only 133k miles on it and runs like a beast.  Does anybody have any idea what we are missing here. I have ice cold heat 95% of the time, then once and  a while for no reason there is some. But that's few and far between. Any help would be very appreciated.