rcpilotken

I bought a mini mag and decided to change the motor, so I bought a brushless A2208/12T 1800KV the prop recommended for this was a 7x3.5 and the esc was a 20amp the battery was a 2cell 1300mAh.
On startup the prop jerks and crackles, when running at peak revs it suddenly grinds to a halt and screeches, the sound is akin to the gears in a gearbox breaking but it is not connected to a gearbox.
The motor also after a few seconds slows down as though the battery were flat but starts up again after closing and opening the throttle ( the battery is not flat)
However after changing the esc for a 30 amp one the motor does not jerk or sounds as though it was shattering a gearbox but still shuts down after a few seconds, changing the battery to a 3 cell one overcomes all of the above problems but the battery is too big for the plane.
I have tried running it without a prop but no difference, have checked the lead outs from the motor but all seems ok, whats going on?

Quorneng

The fact that it does the same sort of thing with both ESCs almost certainly rules out an ESC problem.
If we assume the battery is indeed ok then the problem has all the characteristics of a poor contact, likely a soldered one. It only takes an intermittent poor contact to upset the operation of the ESC either between the battery and the ESC or, more likely, between the ESC and the motor.
You need to make sure all the soldered joints are really secure. Give each a good tug!

A 7x3.5 sounds a bit small when using a 2cell LiPo. This site recommends a 8x4 for A2208/12 motor but using a small prop itself will have no impact on the problem you describe.

rcpilotken

I think i have found the problem, I found a 18amp esc the set up with that works fine, I think the 30 ampesc was shutting the motor down on a small 2s lipo, on closer inspection the capacitor on the 20 amp esc looked to be damaged so I am going to replace it and see if that does the trick