Door opener shuts off at operation

I had to change my springs recently because one of them broke in half, and I got everything switched and the door balanced, and now there are times where the operation is normal, and other times when I hit the button and it comes up like 1-2 feet and shuts off (light and everything).  Sometimes it flickers, sometimes it shuts off completely, but the door stops completely.  If I hit the button again, it goes the opposite direction and then I have to try again.  Its a craftsman 3/4hp.  Any ideas to what it could be.  Whenever the light goes off, if I go up and tap the unit, it comes back on.

It sounds like maybe the drive gear inside the opener is wearing out and may need replaced. The gear can wear out over time and if you didn't notice the broken spring on the door right away the opener would've been doing all the lifting which would cause the gear to wear out faster. Take the cover off the operator and inspect the gear. If it looks worn down and you see white shavings from the gear in the opener cover then the gear should be replaced.

Where is the best place to find diagrams and parts?

Is it a chain drive or belt drive opener? What's the model number of the opener and manufacturers date?

It’s a chain drive. Also I have a question… If it were the drive gear being worn, what would be causing the whole unit to just shut off completely? Model 139.53990D 04/25/05

My suggestion is just a starting point for diagnosing the issue with your opener. If you take the cover off and see the gear is worn it probably is the problem. A worn gear that isn't turning properly may seize up at times causing some resistance that the opener senses which may cause the opener to stop. You can find an owners manual here