On average, the internal power supply in the Time Capsule starts to head downhill at about 3 years of age. This is normal and expected.
Unfortunately, your Time Capsule appears to be right on schedule in this regard.
When the power supply starts to lose its ability to supply full current, the result is loss of wireless speed, and connection drops on both wireless and wired connections.
Powering off the Time Capsule and restarting it often provides some temporary help, but within a few days the TC will normally start acting up again. As the power supply decline progresses, the TC will start acting up sooner and sooner after you restart the device....and...likely within the next few months, the Time Capsule will fail to start up again when you try to restart it.
If you value the data on the Time Capsule hard drive, now would be a good time to archive or copy the contents of the Time Capsule to another hard drive.
Unfortunately again, Apple considers the Time Capsule a sealed unit, so they do not offer or provide repair parts for the Time Capsule. If you could provide the model number of your Time Capsule, we might be able to advise further on the feasibility of trying to find a used Time Capsule and swapping the power supplies out. In most cases though, the cost of repairs exceeds the cost of a new Time Capsule.
