1999 V70: Engine sags below idle when coming to a stop

[volvoscrub]

Hi everyone,

I own a 1999 V70 with 165k miles that previously was showing classic symptoms of ETM failure: random engine surges when stopped, shuddering and slowness when accelerating to highway speeds, and the engine would "dip" slightly below idle when coming to a stop.
I ordered a DIY repair kit for the ETM from ARD Tuning (http://www.ardideas.com/ETM-repair-kit-_p_174.html) and I had a trusted mechanic install it. The kit replaces the flawed sensor design with a new magnetic sensor.

After the repair kit, the first two problems are gone. No more surges, and the car accelerates more responsively and smoothly which is great. However, the last problem has clearly worsened. Now, when I am coasting to a stop or braking to take a turn, the tachometer will almost always "sag" down very low before jumping back up to ~900 RPM (proper idle) and staying there. Sometimes, it gets as low as 200 RPM and the arrow indicator light flashes on for a moment. And very rarely, when I'm stopping the engine will drop all the way to 0 RPM and die! What might be causing this? How can I fix it?

Thanks in advance!