Recap: Just put the engine back in after having to pull it to replace freeze plugs and put on a new tranny + clutch. Also replaced the intake mani gasket which works great, making good vacuum with no more leaking.
The car starts out by revving really high at 3k rpm and then drops sharply and stalls. After a few tries the car will eventually settle down but still surges slowly between 750 rpm and 1200 rpm. The car also vibrates very significantly. The steering wheel is also vibrating, not with the surge of the engine but constantly, enough so my keys make a constant ringing noise vibrating off one another. The car goes into gear and drives forward and backward just fine (amazingly in fact with the new clutch and transmission. The check engine light also came on, never seen it on in this car before.
I'm now beginning to suspect timing belt or balance shaft belt. Both were replaced, and I used the marks to line them up. But all the research I've done points me in that direction. The car vibrates at all rpm ranges. What could be causing the car to vibrate so badly?