STARK

ok fixed for those with same issues was the TFI module.
 

ok fixed for those with same issues was the TFI module.
 

update still haven't changed tfi one from new dizzy wouldn't star.t  went to car club  meeting played up on way in to town late arvo but coming home cold nite ran perfect
 

The only two things which can realistically cause issues like that, is either AC Ripple interference from bad diodes in the alternator rectifier,
(both the Speedometer (transducer) and Tacho (distributor PIP hall sensor) are AC reluctance sensors - ie, they generate a small AC voltage)
 
or a high resistance connection -
a positive or earth connection, common to both the speedometer, tachometer/distributor/coil.
ie, you're losing supply for minuscule amounts of time.
 
Main power relays near the brake booster can fail as they warm up.
 
Don't play swaptronics, look for commonalities between the affected components -
Speedometer/Transducer, Tachometer/Distributor/Coil
 
Is there any pattern to the problem occurrences?
ie, after x amount of run-time, or only in hot/cold weather, etc.
 

those clusters can get a jumpy fault due to bad connections inside, i've fixed a few back 15yrs ago. 
they can get a tarnish/oxidizing on the pins joining the boards and dry/cracked joints.. this may be unrelated to the running of the engine. 
 
i'd have said earths or connections  first also.. seems you have it sussed.. the dash earths are common to have bad connections(where the ECU lives and i think the drivers side has one also? ) 
engine to body earth is very important. 
other than that, 
air flow meters could be troublesome 20yrs ago, 
TFi, Coil, worn bushes in the dissy, worn timing chain.. all factors on EFI crossflows, even air leaks such as dipsticks etc are issues (IAC idle air controller valves could be gunked up)

@Panko explained it's(XF IAC) normally open(or closed, can't remember) and in an E series it's normally closed(or open, opposite/reverse) 
so he made a bracket and ran the hoses a different way. 

cool miss Xfalcon assume normally opened/ closed would be electric side  of things ? have a late throttle body with IAC mounted to it just haven't got there yet lol.

gone over the earths even tinned the 6 wires that were going to one above rh headlight n soldered to the one larger connector didn't fix  fault, but passenger power window goes up n down at speed now lol.
found my paste now just got to get tfi off new ea dizzy haven't got around to striped a screw its always something. 
 

Would that cause the dash to jump all over the place ?

Hi all , have an intermittent fault 
 XF EFI 4 speed sort of late ECU etc, but does it on XF system too.
Is doing my head in, car runs fine then goes to hell no throttle take up misses etc which also causes the XF Fairmont digital cluster speedo and taco to jump all over the place.
running out of things to try  anyone encountered this ?

Scored this off my old man from his Ford days. Some good info in here. Related to the EEC IV (Electronic Engine Control 4) used in XF Falcon/Fairmont/ZL Fairlane/FE LTD. Its focused on the first (leaded) version but generally applies to the unleaded version too.
 
 
Pages 9 & 10 refer to a manual gear switch - I believe this refers only to the four speeds as the T5 wasn't introduced until unleaded models, and the 4.1 never received the "single rail" 5 speed.
 
 
 
 

I went middle of plenum opposite throttle body figured air temp   there would be most accurate and ea series 1 engine wiring used (engine loom separate from main ) puts plug in the right spot.
 

I went middle of plenum opposite throttle body figured air temp   there would be most accurate and ea series 1 engine wiring used (engine loom separate from main ) puts plug in the right spot.
 

I went middle of plenum opposite throttle body figured air temp   there would be most accurate and ea series 1 engine wiring used (engine loom separate from main ) puts plug in the right spot.