I had a case recently where a Dell Inspiron 1545 that went in sleep mode randomly for no apparent reason so I had to isolate the problem:
- I checked the Power settings
- Tested the memmory
- Tested the hard drive
In the end it turned out to be the wristband on our colleagues hand that had a magnet. We also found a spot where if you hold enough the magnet it will put the laptop into sleep. It also seems that this is not an exception for any 1545 dell inspiron.