Hello Brant:

I would first suspect a leak in the intercooler plumbing-- often they don't reveal themselves until they are under boost. Randy Starkie has a great post with a simple tool to test for air leaks.

A bad O2 sensor can cause some bogging, but it will be difficult to sort that out until the other issues have been dealt with. Rich running can be caused by intercooler plumbing leaks, a bad fuel pressure regulator, a bad coolant temp sensor or an O2 sensor. The difficulty is that when the car is running rich, the O2 sensor voltage will reflect it (unless it is dead). The test for an O2 sensor is to see it fluctuate in a timely manner between about 0.2-0.9 VDC with changes in 02 levels in the exhaust.

By the way, let us know what year your 740 is-- it may help to know whether it is LH2.2 or 2.4.

Have you checked your codes?

Good luck!
--
Herb Goltz, Aurora, Ontario, Canada '92 245 w/130K mi, '92 945T w/200K mi