You didn't say if your speedo was wired or wireless. If wired, try routing it as far away from any engine associated parts, clutch/throttle cables, kill switch wiring etc. as possible. Try mounting the CDI on the seat tube or someplace other than the down tube and keep your ignition wiring, blue & black wires, away from the speedo wire.
To answer your question about resistance wire/plug, there has been a lot pros and cons of this discussed here. Some maintain that using TVRS wire or resistor plugs have no effect on engine performance and others will say that a metallic core wire is the best. You can try both methods; you won't hurt anything and it might fix your reset problem. Try using the search feature at the top of this page, type in keyword, 'speedometer' you'll find lots of reading and more ideas and suggestions.
Tom