Please help...1999 Buick Century....3100 engine?

Car is running rough. It started 2 months ago, took it in and they said I needed a lower intake done, they did that along with a full tune up. Got it home and it ran rough after 3 miles, they then changed injector #5, got it back next day same thing, they changed injector #6. Ran great for 2 months. Now its running bad again and engine light is flashing again. We are getting a PO301 code. We changed injector #1 along with the coil and plug #1 again. The first time it was in the shop a compression test was done and was ok. Does anyone have any idea what to do next? It runs and doesn't die, just runs real rough all the time hot or cold. The car has less than 90,000 on it.