Possibly dust buildup in the graphcs card and systems fans leading to overheating?
Do backup your show files and content.
May be worth thouroughly cleaning them one-by-one to see if it makes a difference.
Also ensure that surrounding area is dust free.
Some older Intel Macs only shipped with 2G RAM - so is probably not an issue
(unless there are sticks installed that have failed).
Never recommended to fully enclose any computer - Macs need ventilation and do fail more often in extreme hot environments. Extra forced cooling can help - but can lead to ecxessive dust buildup too...
Some theatre installs have to clean their macs every 3-4 months for this reason...
S