There may be a problem with your crank sensor, and I can't comment on that, but I can say that there was once a time when my '93 SSEi used to idle at low RPMs after starting, and it was due to a dirty throttle body.
On my '93, sometimes (usually only when the engine was warm) the engine would try to idle at 400 or 500 RPMs after initial starting. If I did nothing and let it continue, the engine would typically stall shortly after that, but if I pressed the accelerator a little and held it for just a second or two and then gently let go, the idle would remain in the correct range at approximately 700 RPMs. In my case, the problem was due to a dirty throttle body.
These engines aren't designed to idle at such low RPMs.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles