I just got my intake gaskets replaced and replaced my coolant temp sensor on my 89 riviera. Car is still hard to start in the morning, especially when cold. I replaced the fuel pump a few months ago, as well as the fuel filter, alternator, fuel pressure regulator, spark plugs and wires, ignition control module, ignition coil pack, egr valve,battery, tune up, radiator flush, oil change, and crankshaft sensor. When driving, I put my foot down and experience hesitation and a lot of it when i put my foot to the floor. It almost feels like the car can't get into gear, but I know it's because of lack of fuel. I have bad mileage too, and when I drive over 2k rpms, the engine will shake and shake more violently the higher I go, but always in the same rhythm, shake about a second, then not shake a second and so on. Not sure if I should get an external fuel pump check valve or if I should replace the ecm. Your wisdom and expertise is greatly appreciated!! :)

don't just guess with an ECM. Wor or dirty fuel injectors on one that old often do just that. Have them pressure cleaned first as in this video and see what happens https://www.youtube.com/watch?v=bKGI9N_yWd0