95 T-Bird LX 4.6 w 4 speed automatic. voltmeter in dash goes negative for 5 - 10 seconds after shifting into reverse ?

What do you mean negative? Do you mean it drops below 0?

During normal operation the in dash voltmeter needle is dead center in the gauge.

When shifting into reverse the needle deflects to the left of "normal", or negatively (less voltage) compared to "normal"

I'll put my fluke meter on it this morning and see what the reading is in terms of VDC.

It happens all the time. The car was originally in Park and charging the battery with engine RPM above 500, which is roughly 1,000 alternator RPM. When a load was put on the engine by shifting out of Park into Reverse, a sudden load was put on the engine, slowing it to below 500 RPM. Below 1,000 alternator RPM, most alternators won't "excite" and produce current, and the system is then on battery power momentarily. A fully charged battery is about 12.7 volts, while a normally operating and well-regulated alternator will run in the range of 13.5~14.5 volts. It does sound as if your IAC valve is getting sluggish and needs cleaning/replacement, to respond to engine loading faster than 5-10 seconds.

I tried it this morning. The problem I described did not happen before or after a 12 mile round trip. Will have to keep an eye on it.