Hey Tom,
The problem turned out to be the wiring harness for the O2 sensor...
The harness also carries a main lead to the fuel pump and the exhaust melted it, causing a short....
After rewiring it properly, sealing it with heatshrink and routing it away from any exhaust heat, she's been running perfectly eversince
I found out that this was a common problem among Previa's, Supra's, 4-Runner's and Land Cruiser's...
Rick