Heat issue on my 96 3.1 Lumina

Now that the cooler weather is here I noticed that when I put the heat on in my 96 Lumina with 3.1 L it was blowing cold and after a long while would heat up. I disconnected the heater hoses and ran water through both to flush the heater core and that made no difference. I then tried to bleed the system only to find that once the engine warmed up only steam came out of the valve on the thermostat housing and nothing came out the water pump valve.
The temp gauge reads normal then goes high (but not over heating) for a while then comes down. The motor has been serviced regularly and has never overheated.
I filled the reservoir and every time I warm the motor it looses more coolant and there is an occasional bubbling in the reservoir.
There is no evidence of antifreeze in the oil. Any Suggestions?

Was any work done to the engine prior to this problem , looking into an empty or low radiator are there signs of sludge in the cooling system? Was the system properly bled after servicing? If the return hose is taken off the heater core is there good flow?

There has been no work done to the engine. I am flushing the system as I type this and the heat did come on after I raised the RPMs for a few minutes and seems to be staying on. The bubbles in the coolant return really concern me though. I will check the hoses as you mentioned.
Which heater hose is the return? The one going to the thermostat housing? Both hoses are equally hot.

You have to bleed it when it's filled or you will get wierdness, including running hot and intermittant heater...