A ten shot group - First three shots in an inch, four through six in an inch, but two inches higher than the first group. Seven through ten in an inch, but one onch higher than four-six. Barrel is floated and I checked for contact (with two dollar bills together) after the second and third groups. Same load and POA (at 28x). Checked all gun and scope screws before shooting. Waited one minute between shots (temp 45F) What could be some likely causes of the above behavior or is this a day to be forgotten???

Sounds like the rear action screw.

Sounds like the barrel is heating up ,had this happen on a 7mm rem mag.

Possibly it could be an image displacement problem(mirage) I've had groups move around in weird fashion especially when shooting over snow. The rifles in question were PPC benchrest rifles that I knew were ok. Maybe even the heat coming off a sporter weight barrel on a cold day could cause you trouble. I always cut a pice of target paper and tape on the barrel to avoid the problem when using high power telescopes

I'd check your action screws after you've shot the groups first.
It doesn't sound like a scope going sour. Usually when that happens you get POI shift and opening groups.
My suspicion is your rifle is settling, little by little, into the action bedding. How tight are the guard screws ? Might they not be really tight, especially the front guard screw. Did tighten the front guard screw really tight before you tightend the rear one ? You need to do it that way.
Do you have a middle action screw ? It should be finger tight, no more.
You may also have light contact with the barrel channel when the rifle heats up. Possible, but not likely. Suggest you check barrel clearance with the barrel hot.
Last of all, did you make sure the rifle was in the same position on the bags, and held exactly the same way ? E

Did you have the action out of the stock prior to this. Just a thought as the abouve post that the action may be settling into the stock.