I'd first try new batteries in the FOB's because they are dirt cheap. You can get a package of 10 batteries on eBay for under $6. They are CR2025 batteries, and each FOB requires two. The Owner's Manual tells you how to replace them - it's super easy, just pry open the little door on the FOB.

10 pc SONY CR2025 CR 2025 Lithium 3 Volt 3 V. Battery - eBay (item 300390740093 end time Apr-25-10 22:57:03 PDT)

If that doesn't help I'd replace the main battery in the car. Being it is still the original battery it's overdue for replacement anyway.