Chevrolet Repair: astro overheating?, blown head gasket, coolant tank

Question
87 Astro, engine is rebuilt 94 with approx 60,000 miles. I have had a heat gauge fluctuating between normal and 240 degrees for a while. Gauge seemed to go higher after idling for a while approx 1 month ago so decided to replace sender. I replaced the sending unit and the thermostat. Test drove, gauge went past 240 headed for 260. Went back home, upper hose still cold, so signs of actual overheating ( bubbling in rad of over flow, engine was not snapping like was hot. Since then I have replaced the sender and thermostat again, replaced the water pump which had been replaced in May, radiator was replace mid 2005, flushed system. Have taken to local radiator shop who performed the gas test on rad & said it was ok but did burp a lot of air out of the cooling system. They hooked their computer to the van and it read 185 - 190 at the intake while the dash gauge was reading 235. I had installed a new mechanical gauge for a time to test the dash gauge and the mechanical gauge reads the same as the dash gauge. The last thing I did was the flush. I raised the front of the vehicle to try to bleed the air out of the system and seemed to have some success while the radiator cap was off with the thermostat opening around 230 or so but when the vehicle was road tested again, the gauge still read on the high side of 260 before the thermostat opens and afterwards will fluctuate between 200 and 250. There is no water in the oil, no sweet smell for the tailpipe. I have performed a pressure check as well. The pressure goes up to around twelve pounds when the thermostat opens then goes down to 7.5 lbs. The pressure then begins to build as the gauge rises to around 12 lbs & 250 degrees then temp goes down as pressure releases.
Please help. I haven't a clue at this point nor does anyone I have spoken to. I may be a blown head gasket but really doesn't act like it since the pressure does not continue to build to the point of a boil over and the coolant tank never get fluid pumped into it. I have also pressure checked the cap and that pumped up to 16 lbs then bled down to 14 and held.
Any help would be greatly appreciated.

Answer
Hi Daniel, the problem here is either the head gasket or a crack in one of the heads.  What is happening is there is air in the cooling system and then there is no flow so the thermostat doesn't get hot enough to open. You'll have to remove the heads and either send them out to be checked or just replace the gaskets and pray that takes care of the problem.

Hope this helps

Have a Nice Day                     Mark