The coil and/or rotor could cause this. The coil because its capacity has dropped to a point which gets exceeded by the kv requirements of heavier loading. The rotor if the insulative properties break down such that at the higher kv a new path to ground (not through the plugs) takes place.
Fuel pressure could also be a problem although its less likely to be load related. A plugged cat might also cause this.
Bosch Master, ASE Master, L1
33 years MB technician