This sounds like head gaskets to me. I was an ASE Master Certified tech with L1 and X1 certifications until I chose to pursue electronics repair due to back problems. These GM engines are well known for the intake gaskets leaking both internally and externally but weren't normally the cause of overheating unless the coolant level was low due to loss/leaking. Your description of "bubbling" further supports the head gasket theory since the head gasket (when failing) will allow combustion gasses from the cylinder into the cooling system. These gasses create pockets throughout your cooling system, causing poor coolant circulation, and sometimes a vaporlock that prevents any circulation at all, all of which will result in overheating situations. Sometimes you can see these bubbles when the engine is run with the radiator/or reservoir cap off ( EXCERSISE CAUTION WHEN WORKING ON A HOT ENGINE!!). A local shop should be able to verify the head gasket failure with a test. If they are found to be faulty, the repair would also entail replacement of the intake gaskets, so that issue shouldn't plague you in the future. Good luck, hope this helps.
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
0