Temperature gauge goes infrequently into red

So I pull over and by the time I do it is normal again. Recently checked the antifreeze and when took cap off cold, there was what sounded like a loud air release from the reservoir. After, I drove it a bit and I noticed a little coolant coming from the reservoir cap. Inside the car, you can occasionally here what sounds like fluid gurgling somewhere in the cabin of the car coming from the dash area. I also see occasional moisture in the interior windows now and then. The heat is not very strong and the AC doesn't work in summer. Not sure if all separate issues or related. Car has a lot of miles on it and I don't want to invest much into it anymore. Just looking to keep it running without the frills. A water pump was installed in it about 1.8 years ago because of the over heating issue. And coolant was being added here and there before realizing it may be coming from reservoir cap or possibly somewhere else as well. The thermostat was replaced about a year back. Hoping it's not the head gasket, but looking at all options. Mechanic says it's the heater core, but that does not sound like the issue, as I understand that deals with the low heat issue but not the issue of the engine temperature gauge going into the red. Any advice would be helpful. It seems there are a fair amount of Toyota owners who have dealt with this issue online. Thanks much!