Im a guitarist, im having problems of my left hand position?

when im dealing with things on the high e and low e my thumb likes to sit in the middle at a 45 slant but as soon as like lets say im soloing as soon as i go up or down my thumb sits above close to the low e but not touching..when im improvising or jamming and im taking rythm i may make up a chord using a scale but sometimes when i keep the shape long enough my hand gets tired and i gatta stretch it..for me i ume i need more endurance..when i do quick excersises i dont really worry much on the position of my thumb it takes care of itself i just focus on what speed my left hand should be at with my right..yea im actually messing around on my guitar write now and watching my thumb and its just doing its thing im improvising over scales going up and down the neck and the strings..for finger spreading my thumb likes to reside on the lower portion i can get my figers to spread from one to six like that also when im playing quick my thumb like to reside at the top next to the low e