1994 Chevy Lumina APV acceleration fuel related problem

I have a 94 Chevy Lumina APV, automatic transmission, 3.8, 160,000 miles on the odometer (but we're pretty sure the engine was rebuilt by the previous owner). No warning lights are coming on.

The problem started after the van sat for a few days without being started, and when we did take it out and tried to accelerate up a hill, it surged and lost power and then regained, and then did the same thing again. At a low speed it ran fine, but when you pressed the accelerator, power fell off. During this time, the Check Engine light came on.

First thing we did was check the air filter and found that mice had moved in and brought nuts and other mouse necessities, up next to the air filter, and some may have gotten behind and rolled towards the air mass meter.
We cleaned everything out and that particular problem went away, and the Check Engine light stopped coming on too. We washed the engine off at a car wash and noticed shortly aftewards that when you pressed halfway down on the accelerator while it was in park, the surging started up again - the engine would rev and fall off, rev and fall off. We took it to our mechanic and he cleaned out the fuel filter and cleaned out the fuel system, and also put new plug wires on. That didn't fix the problem, and he's at a loss as to what it could be. We can drive it on the highway; it only does the surging thing in neutral or in park. The Check Engine light doesn't come on at all.

On a recent trip, I noticed that when starting off from a stop, it seemed to lose some power but quickly regain it as I pressed further down on the accelerator.

Sorry this is so difficult to explain. Hoping somebody can shed some light on this for us. Thanks. The mice may have also done a little renovating while they were living under the hood, as in chewing a few wires here and there, removing some of the wire’s insulation. This may have allowed some corrosion to build on the bare wire, changing its resistance value, which in turn would send the wrong sensor data to the computer.
Even though you say that the check engine light isn’t coming on, a sensor’s data maybe at the end of its perimeter; not enough to set a code but just enough to cause your running problem.
The MAF sensor also doesn’t like hard pieces passing through it and if something has hit or is blocking airflow to the sensing wire inside the sensor it could be at fault.
Dan. Thanks much for this response, Dan - I've printed it out and given it to my mechanic, and have an appointment for him to take another look at the van this Thursday. Will post again with whatever happens.

Thanks again,
Gary Are you sure that this "problem" isn't just the Rev-limiter engaging ?