It really should leak all the time, not when the coolant is really hot... In late afternoon heat 2 things happen... 1) the hot air is less dense, so there is less oxygen in it, so your car can't make as much power. 2) the engine is hotter, which increases the chance of pre-detonation
With those two things happeing, your car needs to retard the timing... when it is cooler and the air is cooler, it is ok... let's just say your opti is stuck at a certain timing... it is ok when it is cold, but it is too far advanced in the adverse conditions.... one of the reasons why the LT1 has so much more HP than the L98 is the variable timing... The L98 has to have its timing set up for the worst conditions it can encounter while the LT1 changes it on the fly, so if the better the conditions are, the more the LT1 outperforms the L98