Inconsistent over-revving at idle on Ford 351 EFI (94)?

sorry - not enough info... Its a Ford Econoline 5.8 EFI (1994) with LPG conversion. I've removed the air by-pass valve and cleaned it.. this temporarily seemed to cure the high tickover, but although symptoms HAVE altered (some deviation on tickover at times now) the over-revving (on LPG or petrol) is still apparent, but spasmodic..sometimes idles normally other times 1500 rpm. I've cleaned and sprayed wd40 on all connections and there is no friction on the throttle cable.

TERRY