Check for overheating, particularly of the graphics and CPU. Have you looked at the underside to se if the vents are free of dirt? You can detach the panel, usuall a large one secured by some very small screws, and look at the fan(s) inside. They could have accumualted a load of rubbish. You can manually clean the fan and vents with a small paint brush and cloth. Otherwise, get hold of a can of compressed air from the computer shop and use that. If you do use the latter method, remember to secure the fan with a matchstick or similiar, as the force of the air can damage the plastic bearings. It is also possible, if you dare, to remove the copper tubes you see under there, and check if you need to renew the cooling on the cpu or Graphics chip. It is easier than it looks, --but your decision!