I have the same engine and had TNT problems with the motor cutting out on the internal motor temp relay. It would reset and then run for about 2-8 seconds.
There are two types, mine is a Prestolite. I could get a new motor, not cheap, but was afraid that after the cash out lay that something else would go out, like the pump. Used this as an excuse to get a CMC PT135 and have been happy with it.
This site may have a Sierra replacement. Check the links to OMC parts.
Also, Stone and Son in Houston will rebuild and replace parts. He has a good reputation. Try Googling him.
Good Luck.