The Northstar engine on my 99 Eldorado ETC seems to be shaking noticeably when starting for a few minutes. It is summer now so cold can't be the problem.

There is also enough oil in there, and the gas is from Shell (mid-grade).

The odometer says 72 k miles.

What could be causing the shaking?

mid -grade and you dont get any knocking or pinging? sorry I am no help with your ?

I've also used regular and never had any problems in the past.