HELP!!! Gears Keep locking!!!!

[kasper01001]

ok this happend to me twice today on two seprate Xmods, ok (problem 1) I was working on a S2 red motor to turn, I keep looking at it and it seems that the gears are meshing together but, still everytime I put fresh batts in it would make a skeeking noise then stop!!! so I took it out and just put a stock motor. (problem 2) the Next one was Just when I got a new motor from the mail, it turned when I installed it and then much later it locked!!! I opened it and loosend it up so i'll turn so it did, but then now sometimes i'll go then lock then i'll have to push it a bit to go, then later on it'll just lock like Problem 1 ^ ARGHHH!! I am getting frustrated!!! any help!!!!!????