There is a chance that the problem is not caused by voltage drop on the input but rather by harmonics generated by the switch mode supply. This is a common cause for problems in badly designed power supplies for household appliences.

There are two ways to approach a problem like this. First step would be to try to eliminate the source, i.e. the computer supply itself. Use a line filter on the AC input (they work both ways). Secondly, if your AC input on the transformer of the controller supply is not fitted with a line filter, put one there as well. It's good designers practice to use filters anyway, so learn yourself to think filter when you thing power supply.