2000 Mercury Sable Engine Hesitation

I've noticed over the last week or so that my engine hesitates, sometimes on acceleration and sometimes when my speed is steady. I haven't noticed it on deceleration. It's worse when the engine is cold, but still happens after reaching operating temperature. Over the past two days the hesitations have become significantly more frequent, as in, I feel it several times in ten miles, as opposed to once or not at all in ten miles like it was a week ago. I haven't noticed any strange sounds, rattling, groaning, squeaking, anything like that, but I have noticed that my rpms don't seem to fluctuate when the car hesitates. I've also noticed that sometimes the transmission seems a little sluggish when shifting between 1st and 2nd gear, and 2nd and 3rd gear, and is sometimes a little rougher than usual between 3rd and 4th, and 4th and 5th. However I can't tell if this is because there's a hesitation at the same time of the shifts or not. It does not happen all the time and doesn't SEEM to be an independent problem, but who knows. I can't think of what could possibly be going on that would cause that kind of a symptom without also causing the rpms to fluctuate, either up or down.