Transmition problem but then again not!?!

Okay the problem i have been noticing with my 92 is that when i have the gear selector in 3rd there seems to be no deacceleration at all! But if i put it up into 4th (OD) then it deaccelerates fine. And I have a felling it could be a solonoid etc but not sure. It does read out some code when started from the power light on the dash but ill have to count the amount of times it flashes and see if that tells anything. It isnt a huge deal for me just one of those annoying things like sqeeking brakes if you know what i mean. If anyone has any advise please let me know..Thanks