I have 04 Express 2500 with 224,000 miles it has been well cared for and it runs PERFECT, after about 15 minutes of driving the check engine light begins to flash between 5 and 15 times , its different every time, the motor does not skip, pop, fart ,cough,or hiccup, NOTHING it runs perfect, I am due for inspection and the obd has a P0300 code stored, had it erased and it came back, had mechanic check it out he replaced the intake gasket and gave it a tune up, new plugs etc, no change PLEASE HELP it runs to good to give up on and I cant afford another experiment.

 

Random misfire.
First thing above all. Check your cop's. Coil on plugs. You need to get a shop to check all 8 and see if there is an issue there.
Part 1 -Coil-on-Plug (COP) Test: GM 4.8L, 5.3L, 6.0L, 8.1L.

 

you need to isolate the misfire to a particular cylinder. you will need access to a full fuction scan tool

 

the mechanic that looked at it had a lap top cumputer style scan tool , and drove it for an hour , he saw the same things and the only code he got was 0300, scan tool was unable to isolate the misfire, I'm guessing thats why he tried the intake gasket, thats the only thing he could figure wuold effect multiple cyls, is it possible that all 8 coils are trying to die at once?

 

the gm misfire counter is so good that is will even show accumulated misfire history. I have used this feature with both the oem factory scan tool and a genysis scan tool. If a gm product is misfiring, the data is there to be accessed.

That is how you diagnose this type of concern. Without this information it would be anyone's guess. doing repairs without evidence obtained from proper testing always ends badly. Contact the shop who did the repair and tell them the repair you paid them to do did not fix the problem. A reputable shop will in the least diagnose this correctly for free. If they aren't skilled enough to continue diagnosis go to a dealer.

 

most likely unrelated, but we had an old malibu back in the day with constant missfire codes. We finally had the injectors cleaned and that fixed the problem.

Brew