Engine swap help

I just pulled a very good working 305 TBI out of a 1988 Chev truck 2WD, and put it into a 1987 Chev Silverado 2WD. After hooking everything back up on
the Silverado, the truck fired up right away. It ran great, up until it got to operating temp. When I try to drive it, it starts to backfire through the throttle body, and quits. When I try to restart it it rolls very hard, just like the timing is off. When the engine cools down, it fires right up and runs great again until it reaches operating temp. I know that the Distributor never moved during installation, so I can't see that the timing would be off. Would the computer from the 1988 truck be different than the 1987 Silverado? Or something else. Any suggestions would be greatly appreciated. Thanks.
Mike
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I put a tester on the truck, and I am getting a code 42. Would anyone know what to look for on this code warning?
Thanks Mike

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.