I am not familiar with this type of grinder, but had a look at a number of reviews on the web. After reading them I guess my suggestion would be to give the existing grinder to your mother-in-law and to buy yourself a Mazzer Mini. I use a Mazzer commercial grinder and I reckon its about the best grinder available (....I am ducking for cover know waiting for my comments to be refutted!). Anyway the MM is a couple of hunderd more than your existing grinder, but would be a good investment to you.
Thanks for the comments. The nostalgia of the Kitchen Aid is what caught my eye. While it looks nice I am brought back to the thought as to why it was stopped being made the first time around. Anyway, I'll go about now, smartly into the world of higher end grinders. Thanks for the input.
The acidic taste could be from over extraction. Too fine a grind. Also, coffee houses tend to brew heavier than most people do at home. In addition, the spray heads in newer commercial brewers disperse the water more effectively over the grounds so they get better extraction.
The next time you're in your favorite coffee house, ask if you can look at the grounds in the brew basket before they are emptied. Take a look at how coarse/fine the grounds are (should look like ground pepper) and the spray pattern on top of the grounds.
If you ask nicely and they are nice people, you might be able to take some of their coffee home ground so you can 1.) compare it to what you get out of your grinder and 2.) try it in your brewer to see if you get the same results.
I'd bet it is the grind or the extraction. It could also be the coffee - Central American washed coffees often have good acidity. It is a matter of taste. Again, to diagnose, start with the same coffee you get at your favorite coffee shop to see which of the 3 it is. (coffee, grind or extraction).