The reason your yard smells is very simple. Temperature. Between 2pm and dusk is the warmest pat of the day. If any drainage problems were attempted to be fixed, they failed. Stagnant water results in bacterial growth. These bacteria produce gas when they eat (eat during warm temps.), and the gas smells (gas is harmless). The only permanent soulution to this problem is improved drainage or for the short term (1-3yrs), replace your dirt with acidic dirt.