I have a 99 cummins and it started to cut out at high rpms I though maybe it was the lp wo I did a volt test and it was putting out 13.1 volts checked the fp ar idle its sitting at 15psi changed fuel filter then added lucas additive. Yesturday my radio quit working but still has lights on it no dome light and no power to the diagnostics port if checked all the fuses they are all good. I have been told it could be a bad diode in the alt putting out ac noise and iv also been told bad ecm. I do have an issue with with my positive cables on batteries always getting super corroded not long after I clean them. I need some help here before I go dump a pile of money in this thing.

Well I put the new ecm on and it's working great 

Make sure to still test the alternator because excessive AC noise can still damage the ECM all over again.