Motorcycle Repair: HD Transmission, auto shift, fxst

Question
I am the original owner of a 1985 FXST with only 35K on the odometer.  It has been a great bike and has run practically without problem.

After riding it for several hours recently, I found I could no longer downshift from 3rd to 2nd gear while going over 15mph.  If I slowed to 15mph sometimes it would just "auto-shift" into 2nd gear and when it didn't "auto-shift, I could enter 2nd gear by actuating the shift level again (sometimes with a slight grinding sound).  The bike has no issue with shifting from 2nd to 1st gear once you are in 2nd gear; shifting back up into 3rd gear when the downshift from 3rd to 2nd gear was unsuccessful or up-shifting from 1st through 4th from a standing stop.

I've inspected the linkage and cleaned/lubed all the external parts and looked for something that may be bent or moved in adjustment.  I found no noticable issue and did not correct the problem. What do you believe the problem may be?

Also in your answer I would appreciate some additional advice on how to proceed.  I am fairly mechanically minded and confident working on the motorcycle.  I have done some research on the Internet and I now understand basically how this bike's transmission is designed and works. If the problem you suspect is internal (i.e. fork, dog, etc.) can you suggest to me if this is a probelm I can effect myself?  From what you suspect can I perform the analysis work without having to remove the engine from the bike.  Or do you have to remove the engine to get to the suspect problem?

Answer
First you will have to remove the trans to find the problem. It is internal. You probably have a bend shifter dog or a spacing problem. You will not have to remove the motor to get the trans out, but you will have to remove the primary side of the bike. the trans will unbolt from under the bike. You will need a special tool to do the spacing on the gears so you will not be able to do that yourself.
I would suggest that you remove the trans and have it rebuild at a shop and then install it yourself. You will save some money that way.
Good luck and happy riding
Mike