91 cadillac problem

my cadillac dies sometimes, usually when at idle speed, though it has died on me in the 30 mph range. it acts like the idle is high, but the setting is correct. my mechanic and i were not able to find any vacuum leaks, it just had a tune-up, no loose wires, replaced lots of culprits. what am i overlooking?

A '91??? How many miles??? There are dozens of possibilities. From a failing fuel pump to a clogged fuel filter to a failing ignition component.

What “culprits” have been changed? What was included in the tuneup? What other testing was done? Fuel pressure testing? Has the ignition system been looked at on a scope?

Price an engine computer. Yours is way too old. The high idle is one of the limp-mode symptoms. The engine stalls because the computer can’t adjust.

My first suspect would be a faulty fuel pump relay.