Synchros are a weak spot on the MTX-75, and though usually 3rd gear, 1st is a culprit as well.. I am currently going through the same thing, and as a temporary ease to the problem (note: EASE, not fix) you can start with a tranny fluid cycle..
At those miles, I wouldn't recommend a complete flush, but instead pull the drain plug, and drain what will come out, then refill with new fluid (fill till spill)(redline MTF, others, etc, etc)
drive it for two or so weeks, and then repeat the process..
Do this two or three times in total, and you will have accumulated a "mostly new" set of fluid, and you will notice an almost instant change in shifter feel... Look up the synchro grind in the main page faq for more details.
Ray