M110 - idles okay in P/N - dies in D ??

[Tomatoe_Soup]

Hello All-

recently inherited my grandmother's W114 1976 280.
My grandma calls her 'The Handsome Old Gal' and with good reason: she is in outstanding shape as far as paint and interior goes. (Seen winters but no salt ) Starts all sorts of conversations, which is super fun, however, she stalls out when I move the automatic transmission gear shift from 'P' (or 'N') to 'D'.

Some background:

Drove her this past winter to the Bay Area from Montana (1500 miles) just fine, although she drank up a quart of oil every tank of gas (~250 miles). It pretty much went away about 1000 miles into the trip and then demanded 1 quart per 4 tanks or so. Didn't see any leaks below, so decided to ignore this ... Was having trouble with cold starts (my grandma did too), but had good MPG, at least 20 (60% highway/ 40% city). Then, the rainy season started, and she began to have trouble sometimes, stalling out in while in D while at a stop or just pulling out from a stop. Took her to a mechanic who turned up the idle. Seemed to help but still stalled out too frequently for my taste. Had the mechanic do a minor rebuild on the Solex (replacing gaskets + the carb mounting flange) after he insisted the carb was the problem. (He did the whole carb cleaner spray test). I got the car home, but just barely. He said he fixed a few vacuum leaks, yada yada, but now the car will only drive if the idle is turned WAY up. Said there was something wrong/leaking with the fuel pump vacuum or something but I was little inclined to believe him.

After reading around on these boards, I figured something I could first off do myself was change the plugs and wires. My grandpa insisted this is important, as bad plugs on this car once caused him a big headache. Pulled the old plugs, saw they were in bad shape, changed them out, turned the idle speed on the carb down to something that sounds nicer, and got some improvement with idling. The engine sounds smooth (with a tiny bit of 'puff'-ing occasionally), and the car drops down from the high cold idle to the warmed-up lower idle, but it immediately conks out when I shift into D. The funny thing is, the battery, which never gave me troubles before, now will not start the car...

So, the cold start problem is gone. Yay.
The car starts right away with a jump.
Idles just fine, no smoking out the back or leak-spots beneath.
Really Wants to keep running in Drive, but can't, and then won't restart w/o a jump.

Any suggestions of what I should do?
Does this sound like a timing problem?
A Carb problem?
An electrical problem?
A transmission problem?

I am more determined to fix this myself if I can, rather than pay someone else to make the car run even worse...Or a least have some idea from people who really know and care about these old cars before I venture out onto the 'mechanic market' again. ( I'm a student in the Bay Area, and mechanics around here want $100/hour at independent shops (!))

What do you all think?
Many Thanks-

CJ