Jumping from 4th and reverse

[WJAD81D@prodigy.com (MR JOHN P ELWOOD)]

The transmission in my Mk III Magnette (an MGA transmission) jumps 
out of fourth gear and reverse when I'm going slower than I should 
sometimes, or when I am decelerating.  There are only 45,000 miles on 
the car, so I doubt it's wear.  I'm wondering if maybe someone left 
something out when the took it out for some reason, or maybe when the 
former owner put in t a new shift boot, he might have taken out the 
shifter and left out a spring (like someone did on my B-GT).  Thanks 
alot...

 John