It's very common for the distributors on these Vortec engines to be stripped, and even broken where the cap screws go in, because the distributor body is plastic. It's easy to work around though as the side-exit terminal design makes it easy to use heavy zip ties to wrap around the cap and distributor body to hold the cap down tight. After first establishing the cap and rotor are in good condition, I'd zip tie it down and see if the condition improves.
If the distributor isn't otherwise worn out (check for play at the top bushing by pushing the rotor side to side, also try turning it a little either direction and then push side to side again), I wouldn't consider replacing it over broken screw tabs. It may seem hack-ish to zip tie the cap but if that's all that's wrong with it, I'd run that distributor until it wears out.
It's also worth noting that quality of these distributor caps has taken a dive in the past several years, so if it needs replacing, research here first about what brand seems to be doing well. Lately it seems that the UMP offering with the brass terminals and vent (DC820X, they also have a DCR820X that includes a rotor) is a decent choice, but I'd wait for others' opinions on that as I haven't owned a Vortec truck for a while.
Next up would be checking the fuel pressure, as marginal pressure may result in these random misfires under throttle. Filter may need replacing, pump may be weak, or could be injector assembly/regulator needs replacing.
Richard
