Implant crown loose but only occasionally.

It sounds like the abutment screw has loosened and this is the cause for the wobble. The reason you are able to move it to a position where it feels stable again is because the abutment has a hex shaped end that fits inside the top of the implant. It fits down in the top of the implant like this so it won’t spin while the crew is being tightened. If the screw loosens and backs out enough for the abutment to lift up out of this fitted area, it will allow the abutment to move some until it is positioned so that it seats back down in the correct spot. 
It may seem harmless but it actually needs to be tightened back down tight. This is a simple 30 minute procedure that usually does not require numbing. A small hole is drilled in the top of your crown in line with the opening of the abutment until your dentist reaches the filling material used to close this opening before your crown was seated. The filling material is removed and if it was done correctly there will be a small piece of cotton that was used to cover the head of the screw and protect it from being clogged with filling material so that the small implant screw driver will be able to fit back in the head and tighten it down. 
You will want to see a dentist that is familiar with implants and has the appropriate driver to fit your brand of implants. There is a specific amount of torque that each implant manufacturer recommends for their implants and not all drivers fit all implants so try and obtain the information about what kind of implant you have prior to having this done so that you can confirm that your dentist has the right tools.
After it has been seated and the screw torqued down, an x-ray can be used to check that its seated all the way and then the hole refilled with a new small piece of cotton and filling material.

Hi:Most likely the abutment screw is loose and should be tightened right away before the screw brakes.Best,Dr. Maddahi

The abutment and its retaining screw should be checked and tightened. A new screw is often recommended. Also dentist should use proper torque wrench to make sure it is tightened properly. Please don't function on a loose crown since it can cause potential fracture of screw or damage to internal threads. If this happens, the implant may not be usable.