It's definitely the timer. The retaining pin for the knob was broken inside the cartridge and some plastic bits were rattling around inside. I managed to tap out all the debris and sprayed in some silicone lubricant for good measure but I'm getting the same result, I guess the debris has caused some permanent damage to the gears. At least I know what part to get, there's quite a few options online for under $200 and it's really easy to get in and remove the timer. Thanks for your help!