Unpredictable bogging-down when accelerating

I've been experiencing - not all the time- when I accelerate my car just starts to die- as though it's stalling. But- if I let up a little on the pedal it regains it's power, and I have to feather the pedal off and on to get it accelerate. Even though I get it up to speed it will still bog-down if I try to accelerate to hard. By the way, I've recently replaced plugs, wires, coil packs, timing belts, oil, and filters. What's strange is it stops doing this sometimes for weeks, even months and then for no apparent reason it begins this hesitation first while it's cold, and continues for days when I accelerate and then it goes away again. No back-fire, not popping. Any ideas? It's been a great car for eleven years and 260,000 miles otherwise.