What's wrong with this game loop?

Code:

---

            while(true)
            {
                double elapsed = (int) (System.currentTimeMillis() - lastUpdateTime)/1000f;
                w.fps = 1/elapsed;
                lastUpdateTime = System.currentTimeMillis();
                w.step();
                processInput();
                processTasks();
                gameView.getGamePanel().repaint();
                if(System.currentTimeMillis() - lastStatsUpdateTime > 500)
                {
                    lastStatsUpdateTime = System.currentTimeMillis();
                    w.updateStats();
                }
                try {
                    nextStepTime += frameTimeMs;
                    int sleepTime = (int) (nextStepTime - System.currentTimeMillis());
                    if(sleepTime > 0)
                        Thread.sleep(sleepTime);
                } catch (InterruptedException ex) {
                    ex.printStackTrace();
                }
            }

---

This is supposed to limit the FPS to 60. However, from frame to frame the calculated FPS varies wildly between 30, 60, and 0... but only those 3 numbers, or something very close. I've tried removing everything except the drawing code and it still does the same thing.

Does anyone see the problem? This has been bothering me for months, and I think it's the cause of some slight physics instability.

Not exactly sure what you are doing in this loop. Other than the process the input and tasks, And not sure why you want to limit the FPS.
FPS can be calculated using milliseconds. (1000 / FPS) = MPF. To go from MPF (milliseconds per frame) to FPS.
You just do basic algebra to solve for FPS. FPS = (1000 / MPF)
Not sure if this helps but since no one has posted on this in months i thought id try :P: