Hello!
I have built my own vacuum forming machine, works pretty well. It is decribed here:
http://cs.finescale.com/fsm/tools_techniques_and_reference_materials/f/23/t/151922.aspx
What could help you is one trich that I have found out accidentally. Ihave made my master out of wood, that has been varnished with lacquer to make it smooth. I also had problems with sticking of the clear sheet to the master so I have wiped the master with liquid soap. After that no sticking anymore, even after many weeks, so I recommend you try it!
Another thing would be to fill your plastic master with plaster of Paris or something like that, to make it more stable and more heat resistant, too!
Hope it helps, have a nice day
Paweł