IT can also mean a small leak in a normal vacuum condition is showing up as a failed intake manifold to Cylinder gasket or another condition similar, during high deceleration moments of where intake manifold vacuum exceeds a limit of a gasket so it generates a leaking seal - where the air seal around the various points of entry - made another point of entry either as a poorly retorqued spark plug (unlikely) onto a throttle body to intake gasket seal fail or even a loose MAP or crushed PCV hose - but not as crushed as closed off, more like cracked and allows air in during this high-manifold vacuum condition causing the leaner misfires.
In regard to this, the 30,000miles is about the time when the throttle body should be inspected and even cleaned - but to point to a specific area - this condition is linked to lean mis-fire - all cylinders - random, during deceleration - and so oxygen is getting to the O2 sensors - enough of where the Catalyst is in danger of overheating and further damage of collapse. So, the exhaust system is even suspect as in loose manifold bolts or the output collector to intake of the catalyst - that seal is failing - because the engine can get air in thru the exhaust manifold during these high vacuum intakes - due to the leak in the seal - you did tell us the engine was replaced.
So, did they do their job correctly?
How do I know this - ever hit a deer?
When a frontal impact like what a deer can do - shifts (moves) the motor - even off its' mounts - and if you rolled over that mess - it can also make even more of a mess with the exhaust system and the EVAP/Gas tank inclusive.
So the "put it all back together moments in the shop" one of many things can be reused...sometimes that includes much of the OEM exhaust system - so it just gets bolted back on - then you have another mess with that - and loose bolts are a tell tale sign of poorly reassembled motor and or vehicle - so you have to roll up your sleeves and get dirty to find the leaks.
