Very large workbook now giving incorrect results :(

Grr!

Excel is giving me incorrect results with certain areas of the
workbook. I am using excel to analyse data by essentially counting
instances in which certain criteria are met. I have the ability to
check these results manually by autofiltering my data, and can confirm
now that Excel is giving me completely incorrect results!


Here it the formula in question:


=SUM(((ISNUMBER(FIND("Se",'Supply Data'!$d$3:$d$2000)))*('Supply
Data'!$i$3:$i$2000>$P$30))*('Supply Data'!$R$1=K273))


This formula counts all instances in my raw data (Supply Data) that an
entry has the letters 'Se', has a termination date greater than the
beginning of the year, and is within a certain location. Trouble is, I
can set the current year to be today, which SHOULD garauntee that the
result is zero (as no one can have an entry greater than today since I
am using historical data)... I end up getting 12, and nothing I can do
changes this. After futzing around with the autofilter for many moons
and double checking everything, suddenly the result will drop down to
0.


Is there any reason that Excel is mucking everything up? Any limitation

to the amount of calculation the program can do before it starts
misplacing things? FYI the workbook takes about 5.5 minutes to fully
calculate new data, is 36MB in size, and has 33 sheets. PC is a 1.7Ghz
with 512 RAM.