The techniques you are using are inappropriate and will likely result in incorrect results and convergence problems.
The first problem is in the way you formulate your capacitor. When you calculate C as a nonlinear function of voltage, and then try to convert C to current, you end up with the wrong capacitance and charge conservation problems. You should instead compute the charge analytically, and then differentiate it with ddt() to compute current. This eliminates the accuracy problem. It also eliminates the problem you have having with the simulator, that leads you to your second problem.
The second problem is that you are using an internal node, which also results in two problems. First, an internal node is kind of expensive. Second, it has the wrong tolerances. You declare the internal node to be electrical, but it is not. As a result, you are using the wrong tolerances for that signal, which can result in convergence problems if they are too tight or accuracy problems if they are too loose.
I recommend that you follow the guidance provided in "Modeling Varactors",
http://www.designers-guide.org/Modeling/varactors.pdf.
-Ken