i know exactly what it is.
the mtx-75 has to be drained of its gear oil and refilled. this should be done every 30,000 miles. i jsut did it today. it was having the same issue yours did. and it SOLVED THE PROBLEM.
i got the royal purple synchromax from massive speed system. you'll need three bottles.
jack the car up and put her on stands in the front. and pull out the drain plug (it got an allen head and its hidden behind the dogbone bracket. the head size is 8mm allen key.
drain all of the old junk out (mine was dark brown, shows how long its been in there)
put the plug back in and tighten it to 33 ft.lbs.
then go the to the front of the car, you'll see the same type of plug right on the front of the trans. open that and use the yellow bottle tips to squirt in a whole 2 quarts. then go the third bottle and squrit some in till it starts seeping out the front a bit. then put teh plug back in and tighten that one to 33 ft lbs. and call it a day. get the car off the stands and go for a short cruise and marvel at the smooth shifting.
the reason for the difficult shifting is that the gear oil can properly lubricate the synchros when its worn out.