If you are having temp control problems, put your cuttings under 24 hours of weak blue-spectrum flourescent light in the most stable temp room of your house, well away from any drafts.
If they still droop, mist the inside of the dome more frequently.
And finally, a product like wilt-pruf that is an anti-transpirant does WONDERS.
If your temps are already peaking close to 80, a heat mat may be too much and you could get mold problems. I'm cloning at no more than 68 degrees these days, it's just taking a while. But it can be done. Your issue sounds like water loss.