The key on-off on-off on trick to read codes doesn't work. No 8-8 start of list appears. I get 3 pause 3 pause 4 pause and then the 5-5 end of list. What could cause this? Bad engine computer maybe?

The problem. The truck suffers from a strange stall. Stop at a traffic signel and one out of three stops on the average it will lose idle RPM and cough out. Fuel filter was replaced, gas tank purged and refilled, and the mechanic said he decarboned the heads, but no joy. A cold compression check revealed very high readings on two cylinders. Methinks the decarbon process missed those two so I'll blast them this weekend. That may or may not fix the idle stall problem. Any input on what may cause the idle stall? It would help to have a code reading so does anyone have a clue why the computer won't spit the codes out?