I have an 06 ss with 94k on, I've replace the cat and exhaust (both from zzp) and the car runs horrible. I get on the interstate and I have no power for about 20 miles. No 5th gear and barely 4th, all while the boost gauge stays within 3-5 lbs (never goes beyond 5 lbs even when the car runs fine) during that time. The car shudders really badly when driving and breaking, then all of a sudden, the car seems to figure itself out and it runs fine. I might add that I drain about an eighth to a quarter of a tank within those 20-30 miles. I've checked the spark plugs and all 4 seem to be fine. Solenoid bypass doesn't work either. The car also struggles at higher rpms and like it's starving.