Hayne,
I have an '07 GS450h with 133k miles and the same P2797 code. Lexus diagnostics told me that the Oil Pump Controller Driver is malfunctioned (part# G1167-30021). They mentioned that the part either operates or cools the hybrid system. With it malfunctioning, I'm having the same symptoms as you driving without hybrid. The vehicle starts right up, but with "Check Hybrid System" and CEL Lights both on. I can drive fine for roughly 10-15min. After that, the vehicle goes into limp mode and a few additional lights pop up (VSC, etc.). The vehicle only allows me to drive a max of 50mph. If i keep going for another 5-10min, it limits me to 40mph. Each ~5min after that keeps going incrementally downward to a standstill. If i let the vehicle sit for awhile (1hr), it'll start right up fresh again.
Lexus quoted me $430 for the part. If you add labor, tax, and misc screws, it comes to a total of $1156. I can find the part online for $315. It looks like it is located under the hood on the passenger side. Reference part# G1167 below.
Have you had any luck fixing the issue?