It's time for a pro to look at your set-up. That is way too much voltage drop- I'm guessing that there is another problem waiting to be found. Voltage drop, under load, should not exceed 5%. A 240 v. motor should not be run at less than 204 V.
In theory, it is sometimes permissible to reduce the size of the neutral. For a 30 amp circuit, however, you are required to use a full-size (#10) neutral. Also, the use of different size wires is likely to cause GFI problems.