Just seconding some the comments above. An AC doesn't "create" cold
air from electrical energy, it moves heat from one place to another -
in your case the few inches from the air inside the room to the
"outside" air surrounding the condensing coil located on the back
(exterior) side of the AC. A window or sleeve AC is designed to be
installed with ALL of the openings in the case sounding the condensing
cooling coil exposed to free flowing OUTDOOR air, it's not designed to
operate efficiently when attempting to transfer heat to an enclosed or
semi-enclosed space, or when some portion of the intended air-flow over
the coil is restricted.

Your previous unit may have functioned better by accident (it was
physically smaller, and air flow less restricted), by design (one
possibility is that it was designed for "sleeve" installation, and
provision was made for adequate flow with only a small portion of the
unit directly exposed to the outside), by some combination of both, or
for some other reason (no one here but you here as actually seen your
installation). But it's not uncommon to find the sort of problems you
describe caused by restricted or too-frequently recycled cooling air in
a confined space at the condenser, and that's where I'd start in
diagnosing the problem.

If that is the problem, and you can't find an appropriate (both BTU and
physical) size window or sleeve unit that can be properly installed in
your existing structure, one alternative is a "split" system (the
condenser is a physically separate unit for the evaporator coil)
designed to cool a single room - do a GOGLE or ASK search for "split
room air conditioner", for example:

http://air-n-water.stores.yahoo.net/soapairco12b.html

In addition to cooling spaces that can't be properly serviced by a
"window" unit, split systems have some other advantages; because the
compressor is external and remote the portion of the unit located in
the space being cooled is usually quieter, you don't lose window area,
and you don't have the problem of sealing the opening between the AC
and the window or sleeve.

Michael Thomas
Paragon Home Inspection, LLC
Chicago, IL
mdtATparagoninspectsDOTcom
847-475-568