Today my 1972 240z with 274F Schneider Cam accelerated fine at high rpms but whenever it got stuck in 2 to 3 minutes traffic jam, the idle started to become erratic to a point the rpm fell to 100 RPM and suddenly the engine stalled. I had to pull the choke up or keep the foot in the accel pedal to avoid the car from stalling. The car stalled 3 times, Could this be due to the fact that the exhaust valve on cylinder number 4 was opened to .013 with the purpose of running on 6 cylinder?

I guess i will need to perform another compression test as i am beginning to suspect there could possibly be a burnt or warped valve somewhere.

What directions should i take first before spending money on new valves and seats?