Chances are you have a bad coupler. The coupler (Genie part # 30257T) is made of a dark nylon texture and its purpose is to couple the motor head with the screw drive located within the opener's rail.
While garage door manufacturers are trying to make doors that open faster than ever, they actually use little ingenuity in the development of motors. The motor in my Genie ISD1000/A unit is a 1/2 hp motor that spins at around (I would say) 750rpm. The problem with the motor is that it does not have a slow start feature. When the relay (the door button) is triggered, the torque is instantaneous and the couple takes the brunt of the punishment; especially the side to the motor head.
Here's what mine looked like:
Rail Side of Coupler (normal shape)
Motor Head Side of Coupler (stripped by the torque of the motor)
Confirm this is indeed your problem. Fortunately, this is a relatively easy inspection and subsequent fix. Grab a step ladder or sturdy chair and place it at the bottom of the garage door opener head unit. Remove the light cover lamp; light bulbs if applicable. Then remove the two bolts holding the rail to the head unit. Be careful to hold onto the rail since it may complete come apart from the head unit. At the end of the screw driver, you will see a cylinder shaped device. This is the coupler. Slide it off and inspect it. If the motor side of the coupler is stripped (most likely), just replace the coupler. If the screw drive side of the coupler is stripped, then you may have an obstruction of the screw drive. Try hand moving the screw drive; it should turn reasonably freely by hand. If not and in either case, lubricate the screw drive for the entire length of the rail (use a silicon based lubricant and be careful not to get it on yourself).
Next, temporarily reattach one of the screw of the rail back to the head unit for safety purposes until you can finish the job (you still need to get the replacement part which may take a few days).
Next, you need to find the part. Two ways to get it:
1. Call Genie and complain that your unit has failed. Generally, if the unit is under warranty, they'll send you out a new coupler. If not, negotiate the part for free and pay the shipping. In this case, I would ask for two couplers (for the next time you need one). Genie ships pretty quickly.
2. Google "Genie coupler 30257T". Many online suppliers carry this part including Amazon & sellers on Ebay. I suggest getting two just to have a spare in the event of another occurence.
Once you have the part in your hand, simply slide it back onto the end of the screw drive and couple it to the motor head while turning the screw to line up the slot. Once the coupler has successfully coupled the screw drive to the head unit, simply reattach the rail to the head unit using the two bolts, replace the light bulbs, and reattach the light cover. Test it and adjust the stops if necessary while you're up there. Also, check your door for proper balance. Generally, the door should stay stationary when lifted 3 ft off of the floor (with the opener disengaged of course). If not adjusted properly, call an expert if you have a torsion spring; these are very dangerous since they need to be adjusted while under high tension.
Good luck and I hope this saved you the hassle and expense of buying a new opener.
