Radiator always needs filling, reservoir full

I have a 96 astro van. When i am driving, i have heat. When i am stopped or parked, there is no heat. I check and the radiator is low on fluid. Top it up and wallah the heat is back, however it doesn't last long and i have to top up the radiator again. There is always fluid in the reservoir. I have checked and there is no blockage in the hose from near radiator cap to reservoir. I took it to a mechanic and he said to fush the radiator and add stop leak solution. I flushed the rad, added the solution and prior put in all new antifreeze and water. The engine does not overheat at all. The temp gauge fluctuates and then the engine cools down. I was told by someone that it isnt the thermostat as it is not overheating.
I suspect that it is not holding pressure as it is not taking the fld back from reservoir. Is it safe to drive. Also the top hose when i squeeze it, doesn't feel like there is fld in it. I have topped up the rad about 6 times now. I have ran the engine whithout the cap on and the water rises when the engine is hot. Also when driving i can hear gurgling under the dash when i make a sharp turn. When i hear this, i have no heat, only cold air blowing.

HELP!!!!!!!!!!!!!! Tom

many times coused by the rad cap replace it with gm part

Have replace rad cap and still having problem. Still hear a gurgling sound under passenger side dash. I noticed that the hose for the heater core on engine side is higher than top of radiatior. Think maybe I have an air lock. Any ideas.

I had the same problem with my 87 GMC Safari (4.3 Lt). The reservoir was full and sometimes even overflowing but the radiator was low. It wasn't the cap because it was tested and replaced several times. The engine had over 250K miles on it. Since the engine was worn out we figured out it was a leaking head gasket. You could smell exhaust type fumes from the radiator when removing the cap. It happend mostly when excellerating up hills or heavy throttle. We ended up replacing the engine with a rebuild and everything's great again. I wouldn't put the stop leak in because I've been there... done that. It plugged up the heater core so I replaced it when I replaced the engine.