My 898 side scannng HD has always jumped around. Had about 3 years now. My old 500 series chartplotter worked 10 times better. Some model external antenna. I've tried different apps on my phone and none of them will do anything less than 1mph. I just go by the bend in the line most of the time and what notch I have it on my trolling motor. Like you said. It not worth anything below 1 mph. Even above 2 mph... it still jumps around. I just take the average speed and ignore the jumps.