Disc reading

Hi there,
I have recently had a replacement Dual 2 GHz Power Pc G5 as my previous one's motherboard bit the dust. This one works fine except for two problems:

1) Every now and then the fan will kick into high speed for about 5-10 seconds while the computer is on then go back to normal. This happens regularly but doesn't affect the usage of the computer.

2)It will only read certain DVDs or cds. For example, a dvd I burnt at work on a similar machine can't be read on my replacement one. Also it won't read music cds, and these are legitimate ones. My previous G5 was able to read any disc. Is there any software available to enable my mac to read any cd, dvds?

Just wondering if anyone has any ideas/clues on these conundrums...

Many thanks,
Spud.