Let me guess, you are using an ATI card?
If yes, you are lost On those cards Multisampling can´t be disabled at runtime. This problem is well known to ATI, but until now they didn´t fix it. It might be well possible, they won´t ever fix it.
“Currently, ATI graphics hardware prior to the Radeon 9800 Pro and Radeon 9600 Pro do not support the enable flag for multi-sampling, leaving it enabled in all cases. It is recommended that client correctly set the enable flag for the context as needed but not expect glDisable to be respected for certain ATI hardware.”
(tho, in my testing, that should read “prior to and including”)