Thank you for the replies. Things got busy at work this week - I finally had a chance to do some more testing / layout work this weekend and wanted to provide a quick update on findings.
@Krieglok - checked the wheels and cleaned them (loco was purchased new but may have seen some time on the in store layout - unit was reporting 30 scales miles and an hour run time) - got a little dirt off but nothing major. Cleaned the track as well - no difference in behavior. I did notice when it stalled in the outer rail isolated block moving the front truck side to side would change the signal aspect indicating contact between the outer rails (and common of the locomotive) was intermittent.
@Jon G - resistance between the front and rear pick ups (power / rollers) was between 1 - 2 ohms.. Resistance between the left and right wheel (same truck, wheel set w/o the tire) was 5 - 10 ohms. Resistance between front and back truck wheels (again non-tire wheel) was 25-30 ohms. Both power and ground screws on the truck are the nickel plated type.
I have three block sections w/isolated rail - one in a passing siding and two on the main loop. Removed them all and replaced the Lionel jumpers on the block sections with 14 gauge soldered wires. Engine now performs fine around the layout - all the way down to 3 scale mph. I will switch to IR controllers for the signals. Appreciate your suggestions / advice.