I used to buy about 800 pounds of salvage electronics a week which I would then resell. I’ve seen and evaluated many brands of used older electronics.
For a blender, I’d recommend a Vita-Mix (5200 / 5500), then KitchenAid. Third tier would include Breville or Ninja.
We use a Vitamix 5500 here. A few benefits of the simple design are that the lid has no moving pieces or hinges or locking mechanisms… It’s just a flexible rubber dome that doesn’t crack if you drop it. The blender jug has the blender blade and a very large sealed bearing. You can swap the entire item out as a future service. The motor bases are extremely high quality. I can’t recall seeing any that were ever broken. They seem to rely on solid state electronics and big mechanical switches nstead of fancy displays and microcontrollers and LEDs – most common points of failure on the competitor’s products.
The biggest issue with most blenders is that they are overcomplicated. For example, a ninja blender has detection switches to ensure that the jug is correctly locked to the base and the lid is locked to the jug. If a tab or pin breaks it disables the blender. They could have just designed the shape so that you can’t have the jug halfway installed instead of adding electronics that fail when they get wet or damaged… Bad design choices.
My only warning for Vitamix is to avoid the white color motor base. That color will take on UV damage and turn obviously yellow over the years.
I would not hesitate to buy a well used working Vitamix in the used market. I have seen many units from the mid 1990s and up that run like new.
I’ve fixed a bunch of things but not that specifically.
I’d try spraying the speed control knob with contact cleaner because it sounds like that might be the problem. (Available at any auto parts store in a spray can). Or use 99% isopropyl if you have that available.
If cleaning the connection doesn’t work, you can then replace the whole assembly, the new part runs about $20 incl shipping if you’re in the USA.
https://www.ebay.com/itm/326749491891
That’s a non genuine 3rd party Comparible part. The genuine one is on Amazon for about $60 but I am not sure if it’s 3X as good. Your call.
If I’m not mistaken it would be the 2-wire version but ou might want to look inside and confirm before ordering.
(I’m guessing here, but what I think is happening is that the potentiometer is so dirty it’s like a crackily speaker or volume knob that is sometimes cutting out completely. I suspect it’s wired so that it goes full speed and the knob steps down the speed. So when it cuts out at the knob the motor defaults to full blast.)