Early '08 MBP has a very easily removable battery. They don't have the solid Aluminum bottom like what's linked above in the picture. It's also before they introduced better batteries with more use before charge + lifetime. It might just be a simple matter of age. I don't suspect that my laptop that I bought last Black Friday will be benchmarking as high in three years.
These models are known for their Nvidia chipset failure, even tho your problems arent extreme it might be affected in some way that is leading up to a total failure.
Testing without the battery is also a good idea but if that doesnt make a diffrence then you should consider going to a APR or AASP to get yours tested (the chipset failure has +3 years warranty)