MBP 10.6.4 wont wake up from sleep mode

My MBP is connected to an external monitor and i keep the lid slightly open to keep it running cool (last MB I had overheated and caused battery issues, so was advised to keep lid slightly open). It works fine until it goes to sleep mode, after which it wont wake up. This is annoying as I lose data on documents Im working on as I have to force it to restart.

Is there a solution to this ridiculous issue?