How often can a specter use the level drain
Special Ability?
It drains energy on each attack it does. There is no limit on daily uses.
When your hit and drained 2 levels,
do you get them all back at once when you save?
Or do you have to save each level?
Read the energy drain rules:
If a negative level is not removed before 24 hours have passed, the affected creature must attempt a Fortitude save (DC 10 + 1/2 draining creature's racial HD + draining creature's Cha modifier; the exact DC is given in the creature's descriptive text). On a success, the negative level goes away with no harm to the creature. On a failure, the negative level becomes permanent. A separate saving throw is required for each negative level.
When you regain the level 8th down to 6th,
Can you pick new feats? Then what you originally picked?
Do you roll Hitpoints again or do you stay with what you originally rolled?
You never actually lose levels. Negative levels just give penalties. A permanent negative level just means it requires magic or some other ability to remove. It doesn't actually remove a level. Pathfinder removed the ability to actually lose levels or lose/spend experience points.