08 Odyssey Idle problems

08 Honda Odyssey was running fine, but needed a cat replacement. Pulled everything off, and while we were in it, replaced the front valve cover gasket (had to take off upper intake manifold, vacuum lines, etc.). Put everything back together, bled the cooling system, and this cleared the codes for the cat problem. But, now, it idles rough when in gear (drive or reverse) but stopped (brake is on, car is not moving). Idle fine in park or neutral, but when in gear, it will idle fine for about 15 seconds, then vacuum drops, revs drop and it idles rough, like it wants to stall out. It did stall out once. Put it back into P or N and it goes back to idling normal. Running A/C or accessories in P or N doesn’t affect the idle, only when it’s in gear. No roughness when driving either.

Haven’t check for vacuum leaks yet, which is my next step. I imagine it would have something to do with the repairs since it was running fine before. What else could be causing this problem, since it idles rough only under load.

The electronic throttle body is what controls the engine’s idle speed.