Starting problem returned

first off, i'm pissed. I just put about $300 into my car to get rid of this starting problem, and for the last couple of weeks, its been doing great. But today it was yet another hot ass day and i went to start my car and it would crank, but not start. I think its the main relay, but since the problem only happens under certian conditions, my mechanic hasn't been able to find the problem. I had some wires and a temp. sensor replaced a couple of weeks ago b/c of starting problems. Anyways, i called my mechanic today b/c i was so pissed it wasn't starting and he told me to take it to the shop in the morning and he would try to get it in the schedule tomorrow. He said tomorrow he didn't have to much in the schedule, so he should get to it, but the problem is that tomorrow is the last day his shop will be open until the 14th, b/c he's going on vacation. I hate main relays and heat.