For the last 3 months we have noticed a foul odor that seems to be coming from underneath a bathroom sink. I took apart the "J" and since there was non smelling water and the odor seems to be coming from the pipe in the wall, going under the house. I rinsed it all with a baking soda diluted in a gallon of water. It did help, now the odor only re-appears, on a daily basis, in the hours of worse Texas heat. This bathroom (in our 31 year old home) is only used when we have guests.

The pipe in the wall connects to the same sewer line as the toilet so there is frequently a foul odor in that pipe. The drain pipe from the sink should extend below the wall pipe & then curve back up to the wall pipe. This allows water to remain in the curve & block the odor from the sink. You mentioned the "J". If the sink drain simply comes down to the level of the wall pipe, the there is no way to block the drain line odor from the sink. If so, add the missing "P-Trap" which is the common name for the curved piping previously mentioned.
Your description indicates the trap is installed which leaves 2 possible cures:
1. Extend the trap lower to increase the amount of water left in the trap.
2. When the odor is noticed, run water into the sink to refill the trap.