Drifter

I'd critically inspect the tripod and check for any worn or broken parts of the legs where they attach to the hardware and shim and tighten any place that does not fit snugly together and adjust and tighten every bolt, nut or whatever is holding the hardware on.

Do the leg fall test, raise one leg at a time to a horizontal position and let go and see the rate of fall. It should gradually go down and not just slam down quickly.

The tripod has so many connecting points between the head to the points that dig into the ground that can cause drift. Any screw, bolt or fitting not in adjustment could cause drift.

The old straight one-piece legs for transits and compass had a big brass wingnut adjustment. Loosen them for travel and tighten them before the final level. That and where the footpeg was attached were the only two points of concern back in the day. Today's tripods have many, many points of connection.

If the leg fall test shows quite a speedy drop, even when the leg has been tightened, then the cause may be worn bushes inside the two clamped sections forming the hinge at the top of the leg. They are intended to wear - to protect the hinge castings from damage - and do need to be replaced every few years.

If all is well with the tripod then a common cause of drift is differential heating of the legs in the sun. Apart from a large sunshade there isn't much you can do about that except regularly reset the backsight reading.