Escort 1.8 Diesel reving off the clock?

recently bought an old ford escort lx td, replaced the thermostat because the radiator was not heating up (not sure if this relevant), the car now revs off the clock when the throttle is applied and released (not everytime but intermittantly) the only way to stop the reving is to switch off the ignition (difficult to start next key turn), any help would be appreciated. thanks Rob