i get my scopes on ebay for 100$ they are just little pocket ones though,
if you want to go cheap i'v seen others on ebay, like usb scopes which arnt much
a parallel capacitor would smooth your signal out , but we dont know if that's your problem,
you could be detecting extra background magnetic noise, your sensor may not be detecting enough, 2 very sepreate ways to deal with these,, also how do you know if you management computer is aok?
my first guess is that the problematic sensor(right) is loose, and falling out of detection range when the engine vibrates