try over clocking.. or turn system sounds off.. remove widgets of desktop.
-- remove cssu -- (Yes, I have 4 N900's and non has it.. and they run great.. well 1 is still new in box!)

Here's a thought.. replace the battery??

I'll give the name of a phone, it's the Nokia N'Fakin900!!

Go to you pommy phone shop, and look.. then take your 2+ year old brick and show them that you want something better then it!!

Best of luck