That is a weird one.  A 10-second take on it is:  Alternator current and alternator field current is oscillating and creating noise in unshielded alternator wires/cables.  Replace regulator(s)?   (the alternator lines are mostly ahead of the firewall and noise generated there is unlikely to affect radios, which are not only aft of the firewall, they're also shielded coaxes.)   My 2c worth.  Good luck.

On Jul 24, 2012, at 6:57 AM, Clark Baker wrote:

I have a Garmin G900X in my Legacy. I have erratic EGT readings on all 6 (in flight)   They read around 100F low and are erratic in an about 200F range.

All other temps and reading are fine.

The problem goes away completely when I shut off both alternators. (Shutting off one does not do anything).  No other electrical items have any effect

Note-shutting the alternators off does Not have any  effect on the stormscope or radio noise (there is none) 

We replaced the GEA engine interface box. We replaced all EGT senders and completely redid all EGT wiring from probe to panel. We used thermocouple connectors. We added a second engine ground strap. We checked all grounds.

I do Not get any radio noise and reception is great. The WX500 is clear out to 75 miles.

Garmin has not been much help. Has anyone encountered this or have any suggestions?  We are at a loss.