I have a BLACK+DECKER CM2040 40V Lithium 3-in-1 Cordless Mower, 20". About a month ago, the mower started stalling when on the lawn. If I move it to the sidewalk it will run. On the grass it will sputter and die. I found that if I tilted the mower back toward me, I could keep it running for a while. I raised the height of the mower, that did not help. I thought may be blade was dull. I had the blade sharpened. Still no go. The batteries are charged. I do not want to buy another mower. Your advice is appreciated. Thank you.