There are 3 FSM's in the second topic on this fporum's topic page, here FSM's

Since it is intermintent and the odometer is continuing to clock the miles I would think the problem is in the speedometer itself. Unfortunately the only way to check this is to pull the gauge cluster out.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed