I recently brewed an IPA (5 gallons--all grain). It fermented for 3 weeks, then I added 2/3 cup of cane sugar to the fermenter and bottled the beer--a technique I have done consistently for the past 2 years with great results. The bottles have been conditioning at 70-72 degrees for over one month. When I open the bottles at room temp they are perfectly carbonated (even arguably over-carbonated). However, when I chill them to 40 degrees they become either completely flat or only effervescent. What is going on? How can I serve COLD beer that isn't flat?