1995 Cutlass Supreme LS coolant issue

I had to tell myself to hold the horses thinking I had progress on my heat issue. Ok, I had a small leak from the hose above the thermostat pickup tube that I trimmed and put on a new clamp. Now I have no leaks anywhere but still no heat. I have bled and bled the system and road tested it. It will blow hot heat and then cool. The temp gauge jumps up and the hot light comes on briefly and then goes off and the gauge goes back down to normal. Today I checked it when cold and did discover the level in the reservoir tank was down to the cold line which I took as good because before it sat closer to the hot line. I took off the radiator cap and added a bit of coolant and when it got to the top it gurgled about six times and went down. I topped it off once more, it drank a bit more, I topped it off again and capped it. The hen took it to town about eight miles away and said it was blowing cold air. Is there something I am missing here? A friend said there is a coolant temperature sensor and a temperature gauge sensor on this vehicle that could be the problem. Can anyone help?:shakehead:shakehead

Not a sensor problem that would not affect heat, either the t'stat is bad, or there is still air in the system, or the water pump impeller has failed, and/or clogged radiator.