I seems to have a similar problem on a 2002 alphard 2Azfe fwd 2.4L,as it works very rarely when it is too much raining.Otherwise most of the times it seems to reaching till 20 mph and then jumping to 30 mph and then coming back to 20,even at higher speed.On rare occasions on some days the speedometer seems to be fine.One garage advised to change speed sensor but was of no use after changing.I believe it is loose earth,as it works on heavy rains some how,but not sure where to find this loose earth issue.Any suggestions would be much appreciated.