Air tends to collect in the thermostat housing, so there's a bleed valve there to let the air out. Tilting & moving the engine around to do the clutch may have disturbed some air pockets from somewhere.

Vacuum leaks? And try the "base" idle setup from this thread...
https://www.hondaaccordforum.com/for...e-speed-38734/

 

Shortly after starting
ECT : 88C
IAT : 51c
MAP : 75.0
SPARKADV : occasionally says 0.5, sometimes -1.0, mostly 0.0

O2BIS1 : 0.975
O2BIS2 : 1.035

Tried to bleed the coolant system using the bleeder valve, didn't help. But damn that resovior was twice full before I started messing with it.

Also, I had a little rubber what-looks-like a gasket piece float up in my radiator. Just the one piece, I've no idea wtf it came from.

I think in the 1999 Accord, the fast IAC and regular IAC are combined into one part. There is only 1 intake hole in the throttle body, and the motor dies completely when I cover it completely. Assuming that means there's no significant vacuum leak?

 

To reiterate, when I push the accelerator, she'll rev up and then cut waaaaay back, sometimes dying.

 

If your temperatures are in Celsius, then here is how they convert to F.

ECT - 190 F
IAT - 123 F
MAP - 75 not sure if your units.

ECT seems normal for an engine that is warmed up. IAT seems to be reading high to me, if you scan tool is reporting in degrees C. It should be around the outside air temperature. Not sure about your map sensor, but can you give the MAP reading when the engine is not running?

Is this a V6 or 4-cylinder accord?

It was possible to switch the MAP and TPS electrical connectors on the 90-97 accords, and I'm not sure if the 98-02 accords could also have this happen.

Check the wire colors to the sensors below to make sure they are correct and also plugged in.

MAP: yel/red, grn/wht, and red/grn
TPS: grn/blk, red/blk, and yel/blu
IAT: grn/blk, and red/yel

 

Oh sorry! MAP is kPa, engine is a 4-cylinder.

Yeah, the IAT sensor threw a code at me for too high temp yesterday, but that's the only time I've seen it.

I replaced the fuel pressure regulator yesterday, and now she won't start at all. Even with the old one back on she won't start, so troubleshooting that now

Checked the colors on the wires, look good to me

 

MAP sensor reading when not running is 75.0 kPa. I'm guessing the fact that it reads the same when running and when not running means it's probably bad yeah?

 

Yup, the MAP sensor connector and the TPS connector were flipped. GD.

Still won't start, but that's likely why it was running poorly before. Still attempting to figure out why it won't start now, after replacing the fuel pressure regulator.

 

Yes, the fast-idle function is combined into the IACV starting in 1998.

What do you mean by the reservoir being "twice full" before you started? A bad radiator cap can cause the coolant to move into the overflow bottle but not sucked back into the engine when it cools down. Does the actual radiator stay filled?

Take a photo of that rubber gasket you mention floating in the radiator?