[2001 ZTS]

It's definately electrical. I never had crazy dash lights but the stumbling and lack of speed symptoms I have had. That ended up being the spark plug wires. Since you are also getting light and dash issues I'd say you are right to suspect ground/battery cables. Good luck at the dealer!

There was a battery cable recall (due to fire hazard) don't know if it applied to SPI's but maybe the previous owner never had it done?

EDIT: welcome to Focus ownership, this is the sort of stuff we keep talking about. ;)