What on earth is wrong with my car?

I have a high mileage 2000 Chrysler that is experiencing two unique, and strange problems. These problems will take a bit to describe adequately, so please bear with me.

1. Every now and then my car will randomly overheat. I check my coolant regularly and always make sure that my tank is filled to the appropriate level with the right kind of coolant. Usually, when I first start my car and drive for about ten minutes, the thermostat gauge will rise up to just past the halfway line and then gradually settle to just below it whether the air conditioner is running or not. If I get stopped at a traffic light after driving for a while, my car will gradually heat up until the gauge points to the second from the top line, but will drop back to the halfway mark or lower after I start driving again. When I finally park again, I can hear the coolant gurgling in the overflow tank and smell it in my car. It used to catastrophically overheat until I replaced the radiator cap. Several mechanics have taken a look at it and can’t figure out what’s wrong. I have no detectable leaks, either, but over the course of the week, the level of coolant in my overflow container will gradually decrease to below the minimum mark.

2. Recently, every time I start my car, the entire car will shake and vibrate violently for a moment while the engine makes a loud rumbling sound before settling on a more normal sounding “purr.” My RPMs have also decreased. While idling, my car now sits between 800 and 900 RPMs as opposed to 1500 like it used to. The needle also rapidly fluctuates within that range, but quickly goes up to 2000 RPMs when I reach 50 to 60 miles per hour.

What kind of issues could I be facing here?