Not sure exactly which part of the switch is being grounded above.

ECU logic is that at certain temp range fuel vaporizes inside the lines and causes the rough idle. By raising the pressure attempta to fix that. Ot does that by energizing the VSV. Switches vacuum line to FPR from intake vacuum to atmosphere. This raises fuel pressure to fix idle. Then, vacuum line should be re-connected.

If temp sensor is not sending right temp signal to ECU or if sensor signals proper temp to ECU but VSV does not energize, then problem occurs.

To figure out the above, u cud put a volt-meter to temp input to ECU and also ECU output to VSV and monitor when it happens. However, I confess that I am too lazy to do that, considering the occurence is too rare and happens only for a few seconds after starting.

Manually and MOMENTARILY pulling the vacuum line to FPR but not plugging it exposes FPR diaprhagm to atmospheric pressure as if VSV were energized.
If you have AC, also try turning AC on when it happens, see how idle-up VSV affects it.

There was a recall on 22RE temp sensor that recommends chnging to different temp setting, but i don't know about the 3.0.