Make sure the tool bar is on. Use the 'T' key to toggle it on and off. It is the bar just below the image preview and above the filmstrip. Click on the triangle on the right side of the tool bar. Check the zoom feature to turn it on. Now you should have a slider to zoom in and out and smaller increments.
The Zoom slider in the lower tools panel is not a 'smooth' zoom tool. The "erratic" nature (even with the slider) is that it will only show the 'fixed' zoom sizes and ratios that are also shown above the Navigator Panel.
ie. Fit Fill 1:1 1:2 1:3 1:4.....etc.
Note: that if you select two zoom settings concurrently- eg. [Fit], then [2:1] - Using the SPACE bar will toggle between these two settings. So start with [Fit] then choose your preferred zoom [1:1] now the spacebar toggles these two settings.