Unfortunately, not too sure what is exactly wrong either, but just wanted to run some ideas by you below and sorry if it's too much information.
1. Did you get those adapters from ASUS (eStore?) and have you looked into getting them replaced if they are still covered under warranty (from your place of purchase if they aren't from ASUS?)?
If you are located in the U.S., didn't get ASUS 150w adapter and want to, check the U.S. eStore site
http://us.estore.asus.com/index.php?l=product_list&c=3079, but you may want to check the next idea below first.
2. If your system has been working fine with the 150w adapter before, it's not normal that you should need to use a larger adapter, but if you want to try it, the G75VW 180w adapter will also work (unofficially) on your G74Sx. Hate for you to spend the money on the 180w adapter and not solve the problem, so you may need to decide if it is worth the risk.
Also, wondering if it could be a sign that something is not right with your notebook too, but I am a bit of a worrier.
3. In case your notebook is drawing too much power for your 150w adapter to handle (again, not sure if this is happening) consider fully charging the battery only when the notebook is turned off (which will draw less power from your adapter) and trying to use the notebook only with the battery fully charged if you stick to the 150w adapter.
4. If your notebook is still under warranty, consider sending it back to ASUS to be checked out although not sure if any problems will be found.
If there is something wrong with the notebook though, hopefully they can catch and solve it.
Well, hope this is able to help and we can get more opinions from others.
