Ford Focus Mk1 Electrical Problem

High there, I'm in need of some help. My mk1 Focus is behaving strangely and I wondered if anyone had an ideas as to the cause. I think it's electrical. It
started two weeks ago, a few days after I fitted a new usb cable to the head unit to connect my new iphone. Previously the usb had run to a glove box mounted hard drive. I changed this to a usb to lightning cable to plug my iphone in, when I'm using the car. This might be a coincidence, but I'm adding for context. A few days later, whilst driving the car, with about 100 miles done that day, both wipers wouldn't work on intermittent only. Then the engine started behaving a strangely. It would kangaroo a bit but in gear and at speed. 30-50mph. It got me home and over the next two weeks it has got slightly worse. The engine warning light came on for two days and has now gone off. The boot open light will come on and go off when driving too. Also the speedometer will under moderate acceleration, jump up 20-30mph above the actual speed before settling to actual speed. I have check, cleaned and removed the plugs and leads. They look fine. As does the coil. I have checked and cleaned the battery terminals. Halfords checked the battery and said it's producing 417 amps from a marked 425. I have checked/removed and replaced all the fuses. I have also checked and cleaned the main battery earth. The car has 116000 on it and is only worth about £400, but I'd like to keep it running for another year of so. Has anyone experienced anything similar? Cheers

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