The fluctuation in temperature may have something to do with it, however I am not convinced.
Would you mind posting up the recipe? Copy paste my template and fill out the values that correspond.
Mash time: 70 Min
Mash Temp: ~151F
Sparge/grain rinse temp:
I'll run the numbers and get back to you on estimates.
If you have not pitched yet, the best option is to boil some DME for at least 15 minutes, cool it to yeast temp, add it to your fermenter, give it a vigorous shake, and then pitch the yeast. If you can fit the extra volume in your fermenter, using a rough estimate I would do about 1 lbs of DME (2.75 cups) in a half gallon of water. This should give you (again roughly) 43 gravity points. Thus putting you in the 1.077-81 range. maybe try 2.25 or 2.5 cups to come in a little lower. It will likely change the color of the beer and maybe the taste, but it will bring the gravity up.
On Deck: ***
Primary: Pale Ale (AG),
Secondary: Imperial Wheat (AG), Scotch Ale (AG),
Bottled: Berry Ale (fermented fruit and juice), Honey Weiss (AG), Scotch ale (E), Double IPA (E), Bock (E), Cider, EPA (E), American Amber (E), Hefeweizen (E)