@hectorpez ,Hector, That could be signs of a battery on it's way out or bad connection. Have battery tested, check all connections and wires for not being loose/corroded/damage in the starting circuit/on battery. See the link below as it discusses your issue. I have also attached a down loadable service manual at the 2nd link. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.

http://www.triumphrat.net/thunderbird-tw...

https://trumpettouring.files.wordpress.c...