Bad idle and low speed

Hey guys, new to the forum. Got a problem for ya.

I've got a 98 Cobra. It runs fine while it's cold, but by the time it fully warms up, it starts idleing low and somewhat erratic. Even to the extent of it stalling if I let it idle too long. It sounds like it's just loading up. And then there's the driveability issue. When it's acting funky, it'll be really sluggish and sputtery. But if I press the gas just enough, it makes a very faint sort of a muffled pop from the intake and runs fine. Like the problem only exists at idle and part-throttle. Seems like it's got an excessive amount of fumes at idle too. I read a bunch of different threads, and found one that mentioned changing the thermostat. It didn't suit my problem to the T though. The car has mil eliminators on it, but threw a couple codes for the rear O2 sensors anyway, the other day. I cleared the codes, and they haven't tripped again. The car is just aggravating to drive. I've put 2 different mass-air meters on it, put in new plugs, put in different O2 sensors, unplugged the computer for awhile, and unplugged various sensors to see if they made a difference good or bad.

Nope, no vacuum leaks. I've checked and re-checked. I'm at a loss with it, but I'm about to go tinker with it again. About 10-12 years ago, I had a fox body that did the same thing, more or less. I sure wish I could remember what I traced the problem to!

Today, I tried changing the IAC and coils, along with putting the previous plugs back in. Ok, so here's what I've found. There's that tubular water pipe that goes into the heads. It's got a sensor on both sides. I think the one on the drivers side is for the temp guage. The one on the passenger side is where my problem is, I think. While the car was acting crappy, I unplugged it. The idle quality immediately cleaned up, but went up to 1200rpm. So I figured that sensor was bad. I've got another car that I can swap parts from, to keep me from just throwing money at it. Anyway, I put the sensor in from the other car, and it acted the same way. So I unplugged the sensor, and plugged in original one that I was holding in my hand. The car idled perfectly. So I took it for a spin. The car drove great for a few miles, then started acting goofy again, but not as bad as originally. So when I got back to the house, I popped the hood to check it. The sensor was hot because it had been sitting against the water tube. It seems as though the car ran fine while the sensor was cold, but then as the sensor heated up, it started showing the same signs as the sensor in the water pipe. It just amazes me how bad this is stumping me! lol

I dunno what the "proper" way to fill the system is, but I filled the tank up to the line and pulled the plug in the cross-over tube and filled until it overflowed out of it. After a dozen or so heat cycles after filling the system, the water level remains constant in the radiator. I probably should go ahead and change the thermostat also, to take it out of the equation. Would it be a good idea to take that thermostat bulb off, and go back to a traditional single hose on the bottom with an inline thermostat? The factory hose routing doesn't look too efficient.

When I pulled the sensor out, water came pouring out. So I'm sure there's water in the top of the system, but maybe not enough.

So, I did a search on the filling procedure. When I put a new thermostat in it, I'll follow those guidelines. Still not sure though, the heater blows hot and the car warms up like normal. I'm not opposed to trying any suggestions though. Thanks guys.