failed MAF? (possible not to set CEL).
What year was the dsm?
Possible plugged up exhaust? (cat converter, or muffler, once I had the muffler packing/baffles fall apart, collapse and totally plug up the exhaust causing a almost identical situation on my evo to what you are describing.
Optimally you wanna be able to hooking up a scan tool/datalogger while the problem is occurring. During the low power concern, you want to moniter MAF Hz input, o2 sensor input, TPS sensor input, Coolant temp sensor input, also check fuel trims. You will also want to verify fuel pressure, but from saying the AF ratio is richer then 10: 1 then I imagine its getting enough fuel. This would lead me to believe that u have a MAF problem, or a plugged up exhaust system possibly.
And of course. one of the most common failures, as other people mentioned. Check for boost leaks. this would definalty also cause the issues your describing. Boost leak test with pressurized air and use your ears and soap/water in a sprayer to check/find leaks.
