Setup the TC again.
ie Start from a factory reset. No files are lost on the hard disk doing this.
Then redo the setup from the MBP..
1. Use very short names.. NOT APPLE RECOMMENDED names. No spaces and pure alphanumerics.
eg TCgen5 and TCwifi for basestation and wireless respectively.
2. Use all passwords that also comply but can be a bit longer.
3. Ensure the TC always takes the same IP address.. you will need to do this on the main router using dhcp.. or a bit more complex setup using static IP in the TC. But this is important.. having IP drift all over the place when Yosemite cannot remember its own name for 5 min after a reboot makes for poor networking.
4. Check your share name is not changing.. make sure it also complies with the above.. short no spaces and pure alphanumeric.. but this change will mess up your TM backup.. so be prepared to do a new full backup. Sorry.. but you do need this.
5. Mount the TC disk in the computer manually.
In Finder, Go, Connect to server from the top menu,
Type in SMB://192.168.0.254 (or whatever the TC ip is which you have now made static).
You can use name.. SMB://TCgen5.local where you replace that with your name.. local is the default domain of the TC and doesn't change.
However names are not so easy as IP address.. nor as reliable. At least not in Yosemite they aren't.
6. Make sure IPv6 is set to link-local only in the computer.
There is a lot more jiggery pokery you can try but the above is a good start.. if you find it still unreliable.. don't be surprised if it is you might need to do some more work on the laptop itself. eg Reset the PRAM.. has helped some people. Clean install of the OS is also helpful if you upgrade installed.
Tell us how you go.
