1999 Nissan Sentra 99 Nissan Sentra power and vibration

I've put every one of the 190k miles on my car so I know how it's supposed to sound and 'feel' but I know absolutley nothing about cars.
Lately she seems to be losing horsepower? At times. I can floor it and she'll barely creep along and then all of a sudden take off like a rocket. I've also noticed that she will shudder quite a bit when I'm sitting still idle-ing. Yesterday I thought she was going to stall while I was sitting at a red light. She starts just fine. I've had the spark plugs and wires changed recently, I use Lucas fuel cleaner stuff every 3000 miles and keep her very well maintained. This is the first illness she's shown other than having the original clutch replaced at 175k so I'm a little worried.