Diagnosis
The heating thermostat is likely failing, or massive mineral scale is insulating the heating element so it can't transfer heat to the water efficiently.
How to Fix It
Start with the heavy-duty descaling process I described earlier. Scale buildup is the number one cause of temperature loss. If a thorough descale doesn't fix it, try this: before you brew, run a 'blank' cycle (no pod) into a mug. Discard that hot water. Immediately put your pod in and brew your coffee. This pre-heats the internal plumbing.
If That Didn't Work
The thermostat or heating element itself may be going. There's no user-serviceable fix for that. If the machine is under warranty, contact Keurig. If not, the repair cost usually isn't worth it for this model.
Heads Up
Using distilled or reverse osmosis water can prevent scale, but it can also cause other issues because the machine needs some minerals. Keurig recommends using filtered tap water.