Voltage regulation for Joule amps

I am wondering what if anything others are using for locking the AC voltage for their Joule amps. Based on the built-in voltmeter, my AC varies substantially, when I have the variac set for 89 volts and then the output tubes biased at 27-28, I find that it can be down as low as 82 volts as short while later, with the bias dropping down to 22-23!! I do not have the amp on a dedicated line and can try that next, after the XMAS lights are gone, but I'm betting it is a problem with the power delivered to my house. The difference in price between a Monster 2000 and an Exact Power 15a is almost $1K!! Any thoughts? TIA and Merry Kwanzachristmakah>