I have a 1997 Bonneville SSE.  I replaced the transmission a year ago with a
cheaper used one.

It has started to make a loud popping sound when you give it gas.  It feels
and sounds like splines slipping.  Every once in a while it will catch and
shift normal. When it catches you can gun it and it will shift through all
the gears smoothly.  Once it slips, it does the popping in any gear, forward
or reverse.

Can this be a wheel bearing? CV joint?  torque converter?  How the heck do I
troubleshoot this and narrow it down without having the tear the
transmission out all over again?