Ford - Freestar :: 2004 - Reservoir Too Low On Coolant

My wife has a 2004 Freestar and a few weeks ago it all of a sudden went to red on the temp gauge and there was some steam. So I checked the reservoir and it was too low on coolant. I added the coolant but it still had a tendency to run towards red but turning on the heater full blast made it go down drastically (below 50%). ALso, the heater was blowing cold air. So I changed the thermostat and watched it. It continued to inch up to 70%-80% and then finally drop back down to 40%.

I also had to start adding coolant every other day. I took it in and had a flush done. It eventually blew the reservoir cap off and all the coolant out while my wife was on the highway. I had it towed to a shop and the mechanic said he couldn't find anything wrong with it except the cap. He did a pressure check with the old cap and it wouldn't hold. He replaced the cap and it held pressure. He assumed that was the problem. That was a week ago.

Today it ran up to 80%-90% and then all of a sudden it dropped down to 40%. When we got home I let it cool for 4 hours and then checked the level. It was at the bottom of the reservoir roughly 3-4 inches below the fill line. We don't see any signs of leaking on our driveway. What could the problem be? I checked the oil to see if it may have coolant in it but didn't see any sign.