When standing still and just ideling, I would first considre to service and lube the shifter itself. You will get access by removing the gater boot and the arm rest console, remove the four bolts that hold the black oval plate on the tunnel and then the three special bolts and spring washers that hold the shifter locking ring. Take care not to loose the nylon bush at the lower end of the shifter, if it has not gone lost already.
If so, order a new one and put it on the small ball at the lower end of the shifter. After cleaning the large swivel and it's bearing graphogene greas on all trust surfaces does wonders.
It will take you 1/2 an hour to do this and may be 10$ for a new nylon bushing. Just risk it, changing the synchro on 1st. is an other theme, time consuming and expensive...so worth to try the lube service first.
BTW, I had the same 'problems on my 1975 RD and cured it the way discribed...