I think your first sentence is the most important piece of information. You're practicing again. That suggests you've taken some time off. Good mechanics have to be constantly practiced to maintain. Even playing all the time, I recognize periodically that my mechanics have picked up a flaw and I have to spend a practice session or two of focusing just on achieving good form again. You say that when you focus on it and correct your form, the problem goes away for a bit. It may just be a matter of some more focus and correction and your periods of good form will get longer and you'll spend less time fixing it. Typically when I'm hitting left of target, I notice my elbow is creeping out wide, which causes my forearm to tilt left. Spend some time focused on keeping your elbow in and finishing your release with your hand moving toward your target. Keep in mind, all this focus on your arm will take focus away from your target, so you're accuracy will suffer. That's okay because you're not working on that aspect here. After one or two sessions, go back to practicing focusing on hitting your targets, not focusing on your mechanics, and see if you're straighter.
