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My 1999 gmc sierra misses, backfires and stumbles under load.

While driving the tach will indicate a sudden drop of 500 - 1500 rpms. The truck will buck and shudder while driving ... mostly when accelerating. The truck has a rebuilt engine with less than 30,000 k but no sensors were replaced.
This all seemed to start right after I changed the fuel pump. At that time I also changed the Crank Position Sensor because I was getting a 1435 CPS to Cam position sensor alignment code. That code has not returned.
The only code I am getting now is P0102 MAF input low. I have changed the MAF a couple of times with used MAFs and cleared the code but it returns.
I removed the EGR valve. The mechanical operation seemed fine but I noticed the OEM gasket was broken. I replaced the gasket and reinstalled the EGR. When I started the truck it ran great. I drove home happy, but when I started it the next morning the problems had returned. Can anyone suggest a troubleshooting procedure from here? The EGR is around $400 so I don't want to replace it to find out the truck still runs crappy afterward.
Thanks for any help.

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