A few ideas:
- check your speedo and servo plugs. Sometimes you can get them to short each other out if the part where the metal bits face each other once plugged in.
- try another receiver/transmitter. Hitec aren't the best brand and they tend to be prone to glitching.
- try using caps on the receiver. If you plug a 1000µF and a 0.1µF in parallel on the 3rd channel you'll have a HF filter that should help sort glitches out.