Rev counter problems 3.9 efi

Hi, i have a 3.9 EFI Range rover the rev counter used to play up only working properly over approx 2000 revs, i fitted a LPG kit (disconnecting ECU for a few days) when rewired up rev counter worked perfectly for about 8 months, now it has reverted back to the old problems. i have fitted a new alternator, ECU disconnected for a few days, but it still has the same old problem? Has any one else had similar problems ?
Any advice appreciated.

Col

Hi have you checked the coil because if it's the same as the EFi 3.5's then there is a pick up on the coil sometimes this can become either loose of decayed. Try spraying the terminals on the coil with WD40 or similar