Lets understand skipping for a second. Is it smoking like a frieght train and popping while accelerating or is it bogging down and loosing power and no visable smoke can be seen??

If you can see smoke, this crackling or skipping you may have a too cold glow plug, a clogged air filter or its simply to rich on the top end.

If you do not see smoke then its potentially to lean and has an air leak in the system somewhere in the carb, exhaust, fuel system ect. If that is the case check all points of the engine for wet spots and the check the fuel and exhaust system.

As far as glow plugs i can run a medium cold year round without an issue. Really if you are up to 210 area you could be running the wrong plug.

Try that and we shall see. Oh and btw if its an os speed motor the put a medium plug in it.