When it does return to low idle, after several seconds as you describe can it remain idling steady for at least 10-15 seconds or more? If so, have you tried pinching the fuel line after its idling for about 10 seconds to get an idea of the LSN setting? Aside from an air leak as others mentioned, it could be the needles are not cleaned completely.
If your not familar with adjusting nitro engines i would suggest returning it to the shop that did the work and get them to make it right.