Why an ideal LC oscillator has a highly damped response in Spectre but not in PSpice?
I'm going to guess that it's the integration method used.  Trapazoidal integration is good for oscillators, but Gear integration introduces numerical damping that tend to make oscillators damp out rather than oscillate.

Most simulators provide an option to select the integration method.  Try the same method in both simulators to see if you get similar results.

See here:

"The one area where you should definitely avoid Gear is in LC oscillators. A pure LC tank circuit with an initial voltage on the cap will oscillate forever with trapezoidal integration, but will rapidly decay with Gear integration."