very weird problem still 96 200 mag efi

motor runs superb for first stop on lake then starts acting up. Bogs after 3-3500 rpms if it does run past this it surges bad. Replaced both switch boxes and stator .Also rebuilt mechanical fuel pump and replaced fuel line to engine. Still got problem. Was wondering if it could be ECU or high pressure fuel pump not much left to change. Been working on it for a while. My dad finally got lung transplant but will be in Houston A while. Thank the LORD he is doing superb though. SO get limited time to fix. Very busy between all his stuff my stuff and Grandpas stuff. Thanks in advance for help

Yesterday evening I went to pull the ECM off and found a bolt in the front of the injector cover falling out it was the one with the ground strap that runs to the bolt on top of ECM bracket. Hoping this could be the problem. Only a couple of those bolts were tight the rest turned at least a half a round to get tight.

Could be , but I would test the fuel pressure at the schrader valve at the high pressure pump needs to be 40 ISH...

Jay