crackelly sound on playback

Ive got a Focusrite 56 hooked to Protools 12.4 . after recording through a mic when playback is started there is a crackle sound over the recording making it useless. I've had nothing but problems since upgrading from crashes to when you hit the play button the stop button goes on too? I've been down now for almost 3 weeks and am in serious trouble. can any one help. Protools preferences deleted. removed all plugins including the protools ones , nothing changes. sometimes it records clean then the next time it crackles. have checked all connections used all inserts on focusrite56 directly to speakers and its clean. done cpu/gpu/mem stress checks all good. no hardware problem so it must be between mix control and Protools. thesame thing happened on 12/12.3/12.3.1 and then it freezes when you try to reload file.

Help I'm down to 2 hairs

crackelly sound on playback