Hot weather = clutch chatter?

Just wondering why when it's hot out (~102 today) I get major chatter/shuddering from my clutch in 1st and reverse. It is bad and seems to be mainly when it's really hot. What gives? Could the hot temps outside be causing a slight leak at the rear main? Seems odd but I swear this thing becomes almost undriveable when the temps go above 95. BTW.. clutch has about 70k miles on it and was installed by a local shop. Thanks, JP