To bore sight if a bolt rifle, I pull the bolt, clamp it in my vise(or if not you can use a second pair of steady hands of sand bags, etc or cut a 2 V's in a box to hold the rifle steady), then looking down the bore I pick a spot on the target, paper, wall at 25yards..place a small coloured dot to centre the bore and then just adjust the crosshairs to match. Fire 2-3 at 25 to verify/adjust if required and move to 100 yards firing groups of 3 to make my next adjustments.

Though like mentioned i don't think boresighting is your problem either. Double check the mounts to ensure they are tight, then try some 3 shot groups and taking your time before touching the adjustment knobs - sounds like your chasing the bull!