Here is the situation. I have a engine out of a 2014 wrecked Silverado, so no parts should be bad on the engine. It did not go to the junkyard for an engine problem. It is a gen V 5.3 l83 with 90000 miles on it. I am running this engine on a engine dyno. The first thing I did was try and connect to the computer with a homemade bench harness. I melted some wires during this process because the pinout on the wiring diagram I got off of the internet was wrong. Once I got a diagram from a GM dealer I connected to the computer successfully and deleted VATS and DOD. I took a stock harness and wired in the obd port and gas pedal. The stock under-hood fusebox is being used. The air-box did not come with engine so I milled a place for the MAF sensor in a 4" pvc pipe, and used a fern-co adapter to connect it too the throttle body. I increased the MAF frequency 15% to compensate for this. There is a least a 12 inches of straight pipe in front of the MAF sensor. There is about 18" of 2.5" exhaust after the exhaust manifolds. I got rid of the stock narrowband O2 sensors and put one wideband sensor where the upstream drivers side one went. It is being started by the dyno starter. I did drop the computer on to my knee and then to the floor. My knees were on the ground so it did not go very far. The floor is concrete though. The computer is getting 14.5 v power. 58psi is being supplied with an aftermarket pump. Now here is the problem. It start up on all eight cylinders, but a second later it switches to 3 cylinders. After a minute or so it goes to 2 cylinders. When it runs on two cylinders in runs on 6 and 8. You can hear it when goes from 8 to 3 to 2. I know what the cylinders are doing by looking at injector pulse. At first I did not have a low pressure fuel sensor on the engine. I put that on and there is no change. The map sensor reads zero all the time, but the sensor is good because I tried it on my dad's truck. It has the same engine. I used an ohm meter to check in between the connectors. I used the multimeter to check for power at all the right places on the ecm. The pedal in hp tuners only shows change on one of the app channels. It goes from 6% to 30% pushed all the way down. I checked it with a multimeter also. I think that my problem is in the computer because I haven't been the nicest to it. I believe that I have all the things done that need to be done in hp tuners, but I might be missing something. It seems like nobody has had this problem before. Before I spend 200-300 dollars on a computer and hp tuners credits, I wanted to see if anyone had any ideas. Thanks in advance for any help. I tried to be as detailed as possible.