LTspice diode commutation
There are two problems here: 
1) The grounding situation you have is making the simulation unstable
2) Your constant power load is causing numerical instabilities  

To fix problem 1) get rid of the 10M resistor and put the ground on the load side

To fix problem 2) look at the equation below, what happens when it gets near zero? \$ 1000/0 = \infty \$ and the solver is not going to like that, so its going to try and change the timestep but it still wont be able to solve the problem. You seemed to skirt around the problem by adding in a 10M resistor to ground on the load, but the circuit is still numerically unstable.

 B1 Vdc 0 I={1000/V(Vdc)}


How to solve this? Use a constant power load that the numerical simulation can handle: 


Source: Linear

Here is the way I ran it, I used a constant power load with a B source and also moved the ground to the load: