Lift-Master garage door opener Model 1045-6 1/3 hp

There is a thermal cut out in the motor, usually the motor must cool before the opener will operate again. If the opener only operates 3 times it must be heating up too quickly.
First you opener is only made to do your job for you, which is opening and closing the door. If the door does not operate properly manually the opener will not operate properly.
When the door is disconnected from the opener it should lift easilly, it should basically stay at whatever point you let go of it (midway and fully open) and it should close without slamming down. It does not matter what size the door is!!! If it does not something is wrong with your springs and the door can be VERY DANGEROUS!!
With that being said it sounds more like a static lock-up possibly a loose ground or even a circuit problem in the home.
Bryan

Most residential garage door openers have a max. amount of cycles they can run per hour. Thus the difference between a residential opener which only needs to be opened a few times a day vs. a commercial opener which needs to open 10-20 times and hour. This could be your problem, you may be running too many cycles too soon. This would explain why it seems to work again after unplugging the power. You're resetting the unit. Hope this helps.