I'm Original owner of my 1999 V70 with 300k miles and have an anomaly happening where I need the creative minds to help narrow down what part should I replace and in which order as every time I take it Volvo or to any other mechanic My Volvo doesn’t present herself with my issue. Frustrated but this car's engine runs excellently.

Consistent problem I'm experiencing:
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-tachometer starts to drop immediately below 1000 rpm and starts to sputter and then engine goes limp including power especially when I am slowing down to turn or a stop light.
-Only the battery light and the check oil light and the Up Arrow light displays on the dashboard when car goes limp.
-75% of the time if the tachometer starts to act up, I can safely pull over to stop and put the car immediately in park and just gun it to try to run gas through the engine and then it idles normal to allow me to go my merry way.
-If the car goes limp it will start up 50% of the time with no problem or I have to gun the gas for a few seconds and then it drives normally afterwards. I don't have to wait but today I had to wait about 10 seconds to start again but have to gun it to make sure the tachometer is running smoothly to continue to drive.
-I get NO Volvo codes (courtesy of the last 2 years replacing of various vacuum hoses and other parts)
-I may be driving with my foot on the gas pedal driving on freeway at a constant speed or a city street at 40 mph and then sometimes it feels the engine doesn’t know that I’m giving it gas and may drop by 5 to 10 mph for about a few seconds and then it recognizes that I still have my foot on the pedal and returns back to that speed I was going. (I recall my first experience of this about 2 years ago but didn’t think anything of it until this year where it’s happening often. So I’m thinking whatever is happening this is the major symptom that may be the causing the go limp shut down of the car when I slow down.
-I may pull out of my garage and then I would see the tachometer starting to go under 1000 and a little shaking occurs as if the engine isn’t getting any gas thought I may be pressing on the gas pedal. I put it in park and press on the gas pedal for a few seconds then it idles happily.
Other days, it happens then when I get down 100 yards and decelerate to turn right and the car goes limp with power shutting off.

I have replaced in the past 3 years:
Fuel Pump
Fuel Pump Relay
Both O2 Sensors (May 2014)
Air Mass Sensor (Feb 2015)
Installed latest software in on board computers from Volvo (Aug 2015)
Replaced vacuum hose under manifold (3/2013)
Replaced many other hoses that contribute to Evaporator Leak error codes along with a few smoke test to smoke out any holes from 2013-late 2014.

Had a retired Volvo mechanic check who had the tool/computer to check if my electronic throttle module is the culprit. After checking he said the ETM is not the problem. He suspected the ignition switch may be the culprit but I had that changed in 2008.
I talked to my former mechanic for 7 years in my former state and he suspects it is the Idle Air Control Valve. (When I called Volvo the part was $300. Can I buy an after market one)