I've got a Fitech GoEFI 4 600HP installed on a big block. I'm using an OE style coil and what looks like an OE type electronic distributor. I can't actually tell who made the distributor - it has part no. 1112027 on it. The Fitech is wired to the coil to read engine speed (the blue wire to the negative coil post for you Fitech guys out there). The Fitech is giving me an RPM noise error code (P0335). On throttle it runs like a dream. When warmed up at idle though (in park or drive) the RPMs bounce around constantly and the idle bogs down, pops up...i.e., runs like crap as it tries to compensate for the varying RPM signal. So far I've tried the following in sequence to cut out the noise: 1) Taking the fitech harness apart, spreading out the wires and moving them as far from electrical noise sources as possible. 2) Shielding the rpm signal wire (blue) in braided metal RF shielding from Electriduct with a ground. 3) Shielding the adjacent white wire to the ignition in braided metal RF shielding with a ground. 4) Adding a second braided metal RF shield over the entire wiring harness. 5) Soldering a 10k Ohm resistor into the rpm signal wire (blue) about 10 inches from the Fitech ECU - apparently, this acts like a low pass filter when coupled with the capacitive qualities of the ECU itself (allegedly according to some of the Facebook fitech commentators). No luck so far. My next step is to try replacing the coil, and then if that doesn't work, replacing the distributor. I'm tempted to just skip ahead and get a reproduction HEI distributor from TA Performance to see if that solves the issue. With an HEI, the Fitech blue rpm signal wire will connect to the HEI's tach signal connection. I'm thinking out loud here, but have two specific questions for you Buick and Fitech guys: 1) Any strong feelings about the TA HEI distributor versus an MSD or other manufacturer? The 455 in question is a mild build in my 72' LeSabre convertible - she's a torque cruiser land yacht. Mild cam, short tube headers, Edelbroke performer intake, and the FiTech. 2) Any ideas about what else I might be missing in my troubleshooting so far? I suppose I could try to put my hands on a oscilloscope, figure out how to use it and try testing the RPM signal at the coil, along the blue wire...i.e. try to isolate where the noise is coming in. It's possible its coming from someplace else altogether into the ECU. The reminds me to check the grounding on the ECU with a meter too.