Was doing the routine weed trimming yesterday, and about half way through when I changed out the battery, I noticed when I pushed on the trigger it immediately goes to full throttle. There is no in between. It's either full throttle or off. It is hard to do the trimming in between my roses at full throttle and each time I take my finger off the trigger it turns off and auto feeds. Any ideas what is wrong and how to fix this? Thanks!