87 weekrnder

I have a problem with my vanagon automatic
transmission. Sometimes, when you start the van and
put it in reverse, you have to really rev it to get
the van to move. The trans creates a braking
effecting like I had applied the emergency brake.
Then after about a minute it doesn't do this and it
works fine.

I can also have the van turned off and have it in
neutral. I roll backwards out of my driveway and
start the engine. As soon as the engine starts, the
van stops rolling. When I put it in drive - no
problem. Then is I put it in reverse again - it will
be fine.

Any thoughts?