I have an Echo SRM 210 Weedeater. I can put a spark plug in and it will run fine for a short period and cut off. I then remove the plug and a small piece of what looks like buildup has gotten in between the firing area on the plug. I remove the small piece of buildup and reinsert the plug and it runs fine again for a short period and this same thing happens again. Any suggestions on what may be causing this? Sometimes it works for 30 minutes and sometimes only a few.