I had the same problem on my QP65 and it turns out the field boost current is only applied while the start button is pushed. You can try holding the button a little longer when starting to let the current build or install a field boost kit (0H07850SRV) which provides a current source independent of the start button being depressed. Have not had a problem with 120VAC coming up since the install.
Here's where I bought mine and also a link that has a link to PDF instructions:
http://www.apelectric.com/0H07850SRV-Generac-Guardian-field-boost-uplift-k-p/0h07850srv.htm
http://www.zillerelectric.com/forums/showthread.php?t=1378