Sounds like the synthetic bag is wearing out - it's probably more like a 40-degree bag now, since that's when you start getting cold.

If you want a down bag, go with Western Mountaineering. Most people find their ratings conservative - thus, a bag they rate at 30 might still be warm at 25. My own favorite (at 5'11" and 190 pounds) is the Megalite, part of their Extremelite series. It's rated to 30, but I was comfy in it to 15 degrees last winter inside a tent, wearing midweight long johns (and maybe a Patagonia down sweater; I can't remember.) It might be all the bag you'll need.

Over the years, I've found that if I select a bag that is rated to the coldest conditions I normally intend to sleep in, I'm fine. If it gets colder than predicted, or if I need to push its limits (like that trip last winter), wearing additional clothing is enough to do the trick.

I want to emphasize that wearing clothing in a bag is subject to several caveats. First, it needs to be dry clothing - wet clothing will just proceed to soak your bag and put you in a world of hurt. Second, the bag needs to have enough extra room that the insulation in the bag and the clothing is not being compressed (which reduces warmth); my Megalite is just right for me in this regard. Finally, clothing for additional warmth should be regarded as a safety margin, not routine operating procedure. The 15-degree night last winter was planned, knowing it would be that cold; however, that plan also included camping less than a mile from the car, so I could pack up and end the trip in the middle of the night if I got cold.