to me, it sounds similar to "lean tip-in", except that you're crusing. But if it goes away when you mat it, you might have a temporary lean problem in your part throttle cruise/acceleration circuit. I think 805 might be on to something...many places use different blends of fuel in the colder seasons to enable easier starting (oxygenated blends). If memory serves, those additives alter the mixture requirements a little bit...do you have a vacuum gage to find out where it starts acting funky?
