Up and down when it's on it's back or when you are holding it in playing position? Either way, take the neck off, shim something under there [little wood or plastic shims are common, you can find them in the door section of any hardware store] or I use business card if it just needs a little. If the screws are tight in the neck but slightly stripped on the guitar you might have to fill the holes in, there are many options here like small dowels or toothpicks and wood glue, basically whatever it takes to get the holes small enough to grip the screws.

Hey Tom and welcome to G.com!

On topic I suggest putting a thin piece of foam or a thin piece of wood in there but if its a tight space than I suggest using wood glue or if never want it to move again I suggest Gorilla Glue

its going up and down in the playing position.

Don't want to glue the neck permanently, think ill try the shims. is there a difference in sound between plastic ans wood shims???

Plastic shims might vibrate a bit with the neck and cause a bit of unwanted noise

I suggest a finished wood shim like just throw some deck seal on it and it should be perfect

Tom!

Buy Dan Erlewine's book, The Guitar Player Repair Guide - 3rd Edition.

Product Description

This expanded edition for beginners to experts is a step-by-step manual to maintaining and repairing electric and acoustic guitars and basses. Players learn how to set up a guitar and keep it in top form by mastering basic maintenance. Features an essential DVD that makes guitar maintenance easier than ever. New features include set-up specs of leading players; stronger coverage of guitar electronics, including pickups and wiring diagrams; and expanded coverage of acoustics.