It could be a combination of both. Check to see if the shift linkage is allowing the shifter arm to fully engage in the transmission. Also check for missing or worn-out shifter grommets or worn shift rods. You could disconnect the 1st/Rev shift rod and put it in reverse and back it up to see if it stays in gear. If it still falls out the problem is probably in the side cover and or worn shift forks inside the trans. Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine