Ok, we need a few more pieces of data and sadly they live a few feet deeper in the ground. What you really need is a temperature profile as a function of depth. What is the ground temp 4' down, 4.5' down, 5' down, 5.5' down, 6' down, etc.
I don't know how much of the underlying science you are interested in, but the rate of heat transfer from the hot inside of your copper pipe to the cold outside of the pipe is a function of the temperature gradient (difference) across the pipe wall. If there is only a 1 degree temperature difference the rate of heat transfer (heat flux) is going to be very very minimal. Conversely, if you enjoy a 20 degree temperature gradient, the heat flux is going to be extremely high. When chemical engineers design heat exchangers we work things so that the temperature gradient everywhere is at least 5F. This is so that any given section of the pipe is exchanging a decent amount of heat. You've got (at most) a 6F temperature gradient when the 83F water leaving the Ti coil first enters the 77F ground. In actuality the water temperature leaving the Ti coil will be lower than 83F since it would take an infinite length of Ti to warm it to exactly the temperature in your sump. What we need to do is move the copper loop lower in the ground to a point at which you've got a decent temperature gradient across the pipe wall. This will drive up the heat flux.
What is happening right now is the heat flux across the copper pipe wall isn't as large as the heat flux into the display. As long as this is the case, heat will build up in the system and drive up your temperatures. If we can get the heat flux across the copper pipe wall larger than the heat flux into your display the geothermal loop will be able to maintain your temperature.
I hope that all makes sense. I've made a couple of simplifying assumptions in what I wrote so that things don't get too technical.
