Help with diagnosis PLEASE.....UGH
 

Truck is 99 Auto 4x4 2500. 53 block motor failed on me last fall...I installed a 2001 Motor complete, but used all my 99 Electronics, and Injection Pump as it was only 500 miles old....
I had to install A front mounted Crank tone ring to give the ECM crank signal it needs for the 99 ECM. The truck has been running great since doing this in October. averaging 18-19mpg empty, and 13-14 towing my 28 ft trailer

here is my issue.

5 wks ago the I noticed my Tach jumping around sporadically at speed, It Idles great, with no issues in the tach, and initially it only seemed to act up around the 19-2100rpm range, but since then it has been progressively getting worse... Every time it does this it throws Engine lite and Crank position code.....I thought maybe the Crank sensor was going bad so I bought a New OE unit , with no difference...I do notice it does not seem to start failing till the engine starts to warm up So for the first 5-10 mins of running all is well, but once engine really starts getting close to operating temps, the tach starts failing, then drops to Zero...If I mash on the fuel, sometimes it hops right up, but other times you feel the lack of Fuel going thru the Injection pump as if it is in a Limp mode, and the Check engine lite pops on with crank position code... If the Tach drops off and stays dead for 15-20 secs...then I get Fuel cut thru Inj Pump, and that REALLY sucks at speed while towing....

Looking for Thoughts and Ideas...I hate just parts changing, and If I need an ECM I want to be 110% sure I NEED an ECM.....

Thanks, For any and All advice on this, John

WOW.....204 looks, but no response.....not a good OMEN....:(

Is the crank sensor code the only one you are getting?

So a buddy had same problem in his truck he took it to a shop and all they did was rigged it and it worked for 2 weeks and cost him $800 they wouldnt stand by their work so he took it to another shop and they looked at it and said hed have to replace the ecm due to somebody proding around and jumping wires hope you dont have to replace the ecm hopefully its something small