96 Subaru Legacy 2.5l phase 1 motor 142,000 overheating

This is a problem that has been bothering me since I bought this car about two months ago and it still is not fixed by factory technicians! When I first bought the car the temp gauge kept rising to hot levels and I hear gurgling through the vents. So I took it in to subaru themselves (they know the cars so y take it anywhere else?). It was headgaskets. Both heads were warped and so they machined them and replaced a bunch of stuff (two thermostats etc.) That sovled the problem of bubbles in the radiator overflow/fill tank. They said it was all good. Got it back. Keeps overheating and I hear gurgling in the dash still. I'm a little upset and take it back. Aparently the radiator is clogged and I need a new one. So they replace it and say they promise the problem has been resolved. At this point i'll admit that it was less frequent than before but it still existed. It especially became a problem when driving on the highway for a long period and then stopping or going into city traffic. At that point it would spike up to 3/4 headed to hot. While i'm on the gas the temp is fine. So highway was perfect. Once it starts idling or going slow (about 30mph and below) the temp rises again but not all the way to hot as long as I keep moving and accelerating. If I park it and the temp rises I can give it gas above idle to about 2k rpm's or more and it'll drop to normal but as soon as I lay off the gas, it rises again. The gurgling is really evident on hills and when I hit the gas I can hear it. The heater core is not leaking and it isn't leaving a film on my inside windshield. The fans are both working perfectly. It has a new water pump, head gaskets, radiator, and thermostats at this point. And heater works perfectly as does the a/c. So I take it back really angry and they flush it for free and say "it's perfectly ok now". It is still NOT ok. It happens once every three days. That is the odd thing I cannot figure out. I do the same routine with the car monday through friday and it could be perfect for two days and then all of a sudden, BAM! There it is again, rising temp gauge. At first I thought it was the temperature outside. But we've had everything from snow to 70 degrees and there is no consistency in the problem. I can also smell antifreeze after a long drive but nothing appears to be leaking ( I put cardboard underneath it to check for a few days) and no area on the car is wet with any type of fluid. I know my subarus pretty well and it just makes no sense to me at this point. Any help you can offer would be appreciated.