I am assuming you have attempted CAS2 w/ each stick of RAM exclusively, this should show the one which is locking you up...
As you are aware, some RAM sticks are unable to perform @ CAS2 w/o issues (such as lockups,ect..) But occassionaly you can cheat death by placing the troublesome stick in the 3rd DIMM slot... If this doesn't fix the problem, the following info may be of some comfort...
1) The difference between CAS2 and CAS3 is so minimal, you shouldn't be able to notice.
2) If you are working w/ a Win 9x environment, having 256 Mb RAM can sometimes give you slower responce rates than 128.... (therefore, drop the offending Dell stick, and switch to CAS2)