How many miles are on your car?

I do not know much about the newer models so can only give a general response until someone who knows better will give some answers.

Check your throttle linkages to see that they are not covered with thick skummy oil/dirt that would get more viscous in cold weather. Next it may be that the Throttle Linkage balls need lubrication with some thin oil.
If you do not feel confident to pop the Linkages off to clean them try spraying the Linkage Ball and Sockets without taking them apart with some WD-40 or something similar.

Transmission: Check to see that there is the proper amount of ATF in the transmission. After that inspect the Vacuum lines/hoses looking for cracking or disconnected hoses.
If you find any that need replacement try to replace 1 at time (if you have a digital cammera you can take a pic of where the hoses go before you remove them) so that that you can keep the connections in order.

Also I do not think tapping on the gas pedal will reduce the idle on Diesel Cars. That was for Carburated gass cars where you tapped the pedal to release it from the High Idle Cam on the Carburator after the Engine warmed up some.

I have not noticed one on my earlier model but see if there is an exterior throttle return spring that might have broken or come loose (have you noticed that your Pedal is easier to push down) or is there a spring located inside down by the Pedal?

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84 300D, 82 Volvo 244Gl Diesel