no third gear problem--help

[stanger66]

To give what is going on, My wife bought me the car from my friend and it sat in garage for 6 yrs never started or driven just sat in storage. I have had the car for 2 yrs. Finally got the car started about 1 yr later and had to add 3 quarts of trans fluid to move it. I drove it about around 300 miles. The last time I drove it was in sept. '05. The 2nd to last time I drove it I went to Va. about 120 miles round trip. Took to work next day and no 3rd gear engaging. It has C4 trans. Do you think it sat for so long that fluid dried up and clogged up something. I got a trans kit for it (pan leaking). I was thinking of doing a trans flush at work doing installing the kit and doing trans flush again. Do you think that would work or should I rebuild trans.