Possible battery going bad ?

[None-ya]

Easiest way to explain is when the car got warm - half way through 'normal' - car started to lose volts. I've never had this happen before and it's got me stumped. I will ad that if I pull over, put it in park and give it a small boot full the volts kinda/sometimes come back up, and has only done so when actually driving,

Cars a NL Fairlane 4L