Couple things to look at.

Antenna routing on the rx. Is it up against the esc wiring, or servo wires, or metal control rods? Any can cause an interference problem.

If your tx was too close to the rx antenna during the run up it could cause a problem also.

Loose crystal or damaged crystal in the rx

Low battery voltage in the tx.

Lastly if this is the brushed motor version, you could have a bad capacitor or diode on it that's causing interference.

Oh one other thing, if you were doing this around fluorescent lamps, I've seen them cause interference also on 72's.