yep more information might be helpful here
model number
serial number
what control system is in it (i'm betting EV100)

as for the huge spark under the seat? i'm confused but every bendi i've seen theres a battery under the seat/hood. You may be seeing the spark from the contacts breaking and the flash reflecting through holes where the hood hinges are in the counterweight area in the rear.
but yes, the lurching could be a hung up 1rec but that usually triggers a fault code.
I'd check the contactors and make sure none are welded shut first. The 1A contactor can cause this problem if its welded shut, the system will try to start and run and when it sees the 1A closed it will trip out because the 1A is pulled in before expected, and open the directional contacts causing the big flash.
These trucks are notorious for burning contacts up, especially if the battery is in bad condition or discharged too much.
THEN you can go to the panel and check the rec's.