Aldl connection corrupted data when engine running

Managed to get the winaldl software up and running perfectly trough usb FT232 adapter.
It provides a steady stream of data on all things when car (89 g20) is in ignition on, engine off.
However with engine running the datastream gets corrupted somehow, and values recorded takes on all sort of numbers.

On the usb adapter i can change from 5 to3,3 volts, but this didn't change the results.
So Im thinking it could be one or more of several things?

Bad ground somewhere. wiring harness
Bad alternator, or alternator ground?
Ignition coil, ground
Spark plug wires running to close to the main wiring harness causing interference.

Anyone has experience with this, suggestions would be appreciated :)

BR
/Alex

89 G20 will have slow updates. What is an example of the corruption you're seeing?

as a general rule on any car if the log data is clean when engine is off, but becomes corrupt with engine on,

- try running with alternator disconnected. if situation improves, replace alternator (failed alternator rectifier is introducing AC current into the system)
- try jumper cabling chassis to engine to ECM. if situation improves you have a shoddy ground cable
- try firing engine up in the dark and mist water all around the engine with a spray bottle. if you see sparks, you have a spark plug wire or coil or something with insulation breakdown producing RF that is screwing with your data stream