Offshoreonly.com - 496HO issues at high and low RPMs

I have a 2002 Mercruiser 496 MAG HO w/ ~450 hours on it that's giving me trouble. Here's what's going on:

A continuous alarm (guardian?) goes on when at or near full throttle for more than a few seconds, until I throttle all the way down. This has gotten progressively worse over the last few times I've taken the boat out.
The "check engine" alarm on MercMonitor used to come on at the same time as the continuous alarm, now it's constantly on (plus 2 beeps every few minutes). MercMonitor doesn't report any more specific alarm.
Last week I had someone check the ECU and it had thrown an oil pressure code. Oil level was a little on the low side, so we topped it off and reset the code. Oil level is fine now.
The engine had a little trouble starting this morning. It was a much quieter start than usual and seemed to be struggling until it warmed up after a minute or two.
Slightly above idle it sometimes sounds like it's kind of sputtering or something.
It's stalled a few times at low RPMs, usually when throttling down from just above idle. That may have been due to low fuel tanks though.
MercMonitor is reporting an obviously incorrect oil temp of 32 degrees F. Other gauges seem to be working. Oil pressure was ~20, water temp ~170, water pressure ~3, can't remember the rest.

Other than that it seems to run great around 3000-4000 RPMs. I just got back from a 150 mile cruise with no problems except at the low and high RPMs.

Any ideas what's up? Fuel pressure maybe?

Thanks.