First, you should not have vented your refrigerant. It is normal for the high side and low side to be equal when the compressor is not running. There is also no cut-off for the low side being too high. There can't be, or the system would never start up.

Second, your symptoms sound very much like the system is shutting itself off because it's detected a speed variation between the compressor and the engine. At compressor start, the system compares the compressor speed to the engine speed. When the compressor speed is too low, it is interpreted as a soon-to-seize compressor, and disengages to save the drive belt...thus allowing you to continue driving instead of being stuck with a broken belt. This could be due to a faulty compressor speed sensor, a faulty ACC controller, or your compressor could, in fact, be ready to seize.

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1989 300 SEL that mostly works, but needs TLC