What is the jiggle test? Does it involve Pamela Anderson?

Seriously. My 1991 745 T had almost all of the symptoms you describe, except for the stumble on cold mornings. Mine never stumbled.

After a really lousy head gasket repair job by an incompetent mechanic, I decided to redo the whole shebang last month. After getting a replacement head - the original one was machined down way too much - and a complete valve job on the replacement head, carefully adjusting the valve clearances, and putting the little rubber hushers on the valves, the car now barely makes any noise at all. Except for a politely quiet, but distinctly peppery growl when I get on the boost. She is so smooth it is hard to believe that it is only a four-cylinder engine, and has much more power than she has ever had before. There is a bit of the classic Volvo piston slap when cold, but that goes away pretty quickly (gone in less than a mile) as it warms up.

Maybe your new engine needs a valve job, or at least the valve clearances checked and the rubber hushers replaced.
--
Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)