I had a 95 900se a few years ago that was having a similar problem. the car was down on power, I thought it was misfiring and it sounded just like a subaru. I had no idea what the issue was. I replaced the plugs, replaced all the injectors, I even replaced the ecu and the whole engine wiring harness. the problem persisted.
I wasn't quite sure where to go from there so I ended up cutting my losses. I parted out the car, pulled the engine and set it in my garage for possible future use.
then not too long ago I pulled the engine apart to refresh it and I found that one of the exhaust valves had a chunk missing out of it.