87 Honda Accord tranny problem.

I can pick up an 87 Honda Accord for $150. It's in pretty good shape with the exception of one problem. The transmission.
The car won't start from the D4 or D3 positions (it has D4/D3/2) as the shift pattern. I have to manually place the car in L (2), drive, then manually upshift. It runs smooth as butter doing it this way. However, when I come to a light in D4 and have to idle, you can feel the tranny slip out of gear and I have to manually shift to L again and repeat the shifting procedure. It also does the same thing when you go from Reverse to a forward gear (I have to shift to L, then manually upshift) Does anyone have any suggestions as to what can cause this problem. I've never owned a Honda before, so my experience with them is very limited.