Volvo 940 Turbo Wagon '95 Shuts off

While driving full speed (55 mph), car shuts off. First RPMS drop to 1 - 1.5 (like being in neutral) then car shuts off – no sputter or cough, no rattle or puff of smoke, just kaput! If I try to restart immediatelly (after coasting to side of road) it turns over but won’t start. Wait 60 seconds and it starts and drives normally. Have had it looked at, checked distributor cap, plugs, ignition coil and applied dielectric grease in case moisture was wreaking havoc in that area, but cannot find source of problem. Probably coincidental but it may happen more often in hot, humid weather?

Can anybody help? Thanks!

It could be many things, but I’d start looking in the fuel direction because it is checked fairly quickly.
Maybe the fuel pump relay or fuel pump is having some sort of issue. Gas actually cools the fuel pump. It could be that it is overheating, if the tank is near empty (is that the case when this happens, btw?)
Bring some starter fluid with you and squirt it into the air intake. If it seems like it wants to start, you have a fuel delivery problem.

Edit: do all the dash light come on when it stalls or does everything go dark? That actually should have been my first question.

The fuel pump is relatively new – and the car acted much differently when the old one was going.
All the lights come on when it stalls.
It has stalled with various amounts of fuel in the tank, anywhere from 3/4 to 1/4 tank.

It has to be fuel or spark. That starter fluid trick will help you determine which one is most likely the problem.
Another way is to put an inline spark tester in series with the spark plug when this happens and then try to start it. Those spark testers will flash if spark is present.

Had similar problem with a '98 5 cylinder turbo motor in a V70XC. It was a bad mass airflow sensor in that case.