Peter
On my Twincom, the readings on one side are rock solid, and on the other I see some sawtooth variation. It’s a long time since I looked, but I recall a similar magnitude/periodicity to your report. I’ve always assumed an obscure interference or earthing problem and left well alone. Twins are good for cross checking and also for controlled experiments!
Interestingly, in terms of response time, the JPI is more damped than the old analogue EGT. We sometimes get vapour in the fuel lines, particularly with the tip tanks in certain temperature conditions. You see the fuel flow dip and then rise – but without hearing anything or seeing the JPI move – but you can see a little twitch on the analogue EGT. The fuel flow always stabilises after a while in the cruise.
Alan
I think JPI are running what is called a finite impulse response filter – a fancy name for taking the last 10 (or whatever number) ADC readings, adding them up, and dividing the total by 10. So you keep the most recent 10 and run this filter on them.
When a spike arrives and trashes one of them, it takes 10 sample periods for the trashed value to work itself out of the output reading.
The interference seen in the video shows that sort of thing. The strobe pulses are very brief and would trash only one reading.
What they should have done is tested each reading for sanity, and discarded it if it is say more than 20% above/below the current output value.
..it’s like a CD player with a scratch on it..it get’s it right until something screws up the ADC sampling…DME, XPDR pulse…well my take..as the JPI are known to have fast probes/converters maybe the ADC converter get’s messed up by an EMI pulse… 
This is one of the “problems” with modern instrumentation. A variation like this, where we are talking about a variation of less than 1%, you would NEVER pick up on a analogue instrument.
It is one of the things I dislike about the “glass cockpit” modules, with the Airspeed and Altitude readout. When handflying, it jumps from number to number, which can be distracting, where as on an analogue gauge the needle movement would be very small, and more intuitive.
RPM readout on the EDM models as well. It can easily jump plus or minus 10 rpm (the smallest increment). On the analogue RPM gauge you would never see that difference.
Sometimes modern instrumentation is overaccurate. Then it becomes a distraction.
Wing strobes could indeed give same type of interference, could be the case. Try to seperate wiring as far as practical, or add shielding.
The orginal wiring indeed lacks shielding (or twisting) which makes is prone to electro magnetic interference. As you can’t change this wiring for shielded wiring (special thermocouple wiring), you could only add shielding in the form of braiding. This is what I had done and solved the problem at the Saratoga. This AA59569R36T0500 MIL spec braiding is tinned copper braid. It requires to get the wiring disconnected, to be able to get the braiding over the wiring. Connect the braiding to ground for maximum efficiency.
A very good point about the transponder or DME.
My memory is not good, but I do recall having visited this issue before, a long time ago. So I went through my flight notepads (which I have all the way back) and 2 years ago I tracked it down to the wingtip strobes!
The thermocouple wires are not shielded – as is normal in GA wiring for these instruments.
Fuel flow fluctuation changing the mixture?
If I thought it was that, I would not fly the plane at all. I would immediately get the fuel servo overhauled. Anything looking like a fuel system issue is a total no-fly for me.
Check for good ground connections, and test with transponder and DME off. Transponder and DME are sources of high RF peak output which can easily be pickup by these wirings, especially as the instrument impedance is high and the signals are very weak, just some millivolts.
Also be sure that it uses engine ground only, multiple grounds, or single airframe ground or ground bus is not suiteable for this instrument! Multiple grounds may form loops which act as pickup for noise.
On a Saratoga I had to use additional shielding to prevent interference.
Fuel flow fluctuation changing the mixture?
It happens only sometimes.
A/D converter sampling ?
