Your 60CSx IS a high sensitivity GPSr and should be nearly as accurate as any of the new ones. "High sensitivity" means that the unit is WAAS enabled, and the 60CSx has this, although you should check the settings to be sure it is actually turned on. Also, make sure you have the latest firmware installed on your unit. One thing I have realized, though, is that Garmin's firmware versions are sometimes a bit flaky from one release to the next. Another thing to consider is batteries. Sometimes rechargeables need to be replaced, too.
Newer is not always better. I love the features on my Oregon 550t, but it sends me spinning in circles sometimes just like any of the others. Sometimes its the weather or the satellites themselves that mess things up. Try the firmware and/or batteries on yoour unit before getting a new one.