Please excuse me splitting this in separate posts as I am doing it in between jobs. Though an earth strap could be the issue, with it being the common link to all the onboard electrics for the vehicle I think it would affect other modules too if it was the cause. Having said that it does make sense suggesting cleaning the earth points from battery to chassis leg to the gearbox. Also a must check is the battery voltage from the +ve terminal of the battery to any metal part on the engine for -ve. It should give 12.3 Volts to 12.6 Volts, 12.6 V the better, and that likely rules out bad earths. Here I am assuming that the battery is not tired and holding up to cranking power.
If the original poster has access to and can use a multimeter and even better with a breakout box too then we have to move to testing the CAN BUS system voltages and resistances.
This all leads me to suspect a faulty airbag module from the symptoms and my previous experience on my own car, but before starting to throw money and parts at the car I think it is wiser to move step by step.