I cannot give you any definite information as to the cause of your problem. You say your home is older and the problem only occurs after a heavy water usage. Perhaps your field is small in an older home and not draining as well as it did. When the water fills the field after heavy water usage, then the stored water may be forced back toward the house and has saturated the walls of the basement giving an ordor. 

When the tank was pumped, were the baffles inspected? Probably the best suggestion I could make would be to probe the drainfield to ensure that it there is not standing water in the stone. This would ensure that there is capacity in the field to store the water during heavy water usage. If the field is dry and the baffles are in good shape, then the next thing to do is to investigate the ground around the basement where the odor is coming from.

Good luck.