Before Warlock gained the Soul Gem he demonstrated vast powers in both cosmic energy, psionics, and physical attributes. However, once he gained the Soul Gem from the High Evolutionary, his psionics almost immediately ceased, as did his amazing physical abilities. As time passed, he called less and less on cosmic energy as well. Currently he requires a Karma Staff to even produce effective amounts of this energy. Why?
The soul gem has demonstrated its ability - and desire - to overtake Warlock's body and control him entirely. In the past, Warlock has been able to take parts of souls without the whole thing. The gem is also doing this to him, often making it lethal to remove the gem unless it wishes to be removed. If the gem is indeed constantly sapping a portion of Warlock's soul, then it would explain why his abilities to be damped.
Since the Karma Staff is an extension created with the Soul Gem it would also explain why he can regain his abilities by using it. Obviously the staff it a conduit of sorts to the powers the gem is inhibiting from him. Since Warlock does not often use his psionics or other heightened abilities, it is probably taxing to force his stolen soul portions back under his control.
As such, it becomes clear why his abilities can fluctuate so much depending on the situation.
Affinity to the Infinity Gems:
Warlock has demonstrated a close affinity to not only the Soul Gem, but to all of the Infinity Gems. He himself claims that he is unsure of why he holds this close relationship to the gems - but it has allowed him to perform feats no one else could possibly.
Among these was causing a disharmony among the gems while another used them - and preprogramming commands into the gems to be executed after certain events occur.
The Soul Gem has also stated that Warlock is a host unlike any other it has had - one it could not utterly subjucate to its will. Warlock has also spoken with the gem on several occasions. To Darklore, the Soul Gem said it could reveal even more secrets than it did to Warlock. This line is important in that it clearly establishes the gem has in fact shared secrets with Warlock. While these conversations are not found in any comics - it is clear they existed.
I would guess that the Soul Gem taught Warlock something of the nature of the gems, including how to control them in ways others could not. Adam Warlock was the quickest to adjust to Godhood (The Magus did not possess all six gems) - this may be another indication of things the Soul Gem has taught him.
Warlock is also one of the few beings to enter and escape Soul World on a regular basis. Within the gem (no matter who controls it) Warlock has control of his own abilities and effects of the gem. Since he was able to expel beings from Soul World and back to their bodies - it is presumable that he could also take souls from within the gem. Warlock already is closely tied to the gem even in death, I imagine he will only become closer and closer to being 'one' with the gem as time goes on. At least until it finds another host it is more favorable to. Judging by the number of hosts it has had throughout time - it is a fair guess that this could be millenia in the coming, if ever.
