Sounds like a synchro, but you might find that an oil change might help, with a decent quality oil like Motul Comp FF.

Also, I don't care what anyone says, Redline LWSP is known to fix mild synchro issues....but you MUST change this oil regularly (every 10-15k) or it can damage your box in the long run.

Get the oil dropped and see if there are gold flakes in it. If so, this is synchro material.