98 Tahoe code 300

Hey everyone
I need some help with my 98 tahoe 4x4 with 140000 miles that throws a 300 misfire code.To start ,this has be an ongoing issue for about 6 or 7 years. I started by replacing plugs,wiries,dist.cap all a/c delco parts with no success.Then I replaced spider and fuel pump still no success. The truck runs fine if your just driving around in town and even on hwy but below 70 miles and hour. Like I said the truck runs fine until you put it under load then it will start to shack then the light will come on and it will level out,if you keep your foot in it,it will loose all power and start to decrease in speed until you let out of it. I had a friend put a scan tool on it and checked most senser and what every else and as always he said doesn't make sense. He did say the speed senser was bad but I've talked to others about that and they said if speedometer is working then the senser is ok I don't know. If anybody has had this issue please let me know.

Thanks