Misfire code, loss of power on acelleration, shudders

Volvo V40 2000 with 97K miles. Going on a trip, car trouble literally five miles from entrance to Glacier National Park after driving ten hours over two days. Car drove well until I was passing a 60mph car I tried to acellerate to 75 and the car shuddered heavily, misfired, check engine light flashes. I pulled over. Let car rest, then turned around to find garage on a Sunday. Local small mechanic checked and cleared codes. Misfire cylinder two. He checked the air filter (dirty - looks like mouse shredded something), checked spark plugs, looked clean except for cylinder two which was slightly brown, but he said fine. Checked spark plug gap, was correct. He said it could be bad gas. PUt in a bottle of Heet. Drove car to park and on Going to the Sun Road, a 25mph winding steep road to 6500 ft elevation. Car was ok at slow speeds. Next day, Monday, drove to European mechanic one hour away. Checked code. Misfire cylinder 2 again. He replaced spark plugs. He suggested may be coil, did not have in stock. HE test drove. Said car was fine at 100mph (!). "Blew it out". Meaning going fast to clean out the fuel system? Said felt like fuel problem not coil. Added bottle of Techron. Left and continued on my trip, drove a few hundred miles, slowly and without accelerating fast. When pressing hard on gas pedal, car will misfire, shudder, go slow, flash check engine light. But able to go at slow even speeds, gradual acelleration to 50-60 mph, keeping rpms at 2000-3000, and downhill just fine. Manage to limp through the week like that. Problem still there, learn how to avoid the misfire. Next monday. On trip home. Stop at another mechanic. (Tried to go to volvo dealer, they are too busy) This mechanic checks codes. Misfire cylinder two. Checks pressure. Slightly low, but OK. Notes that last mechanic put in wrong plugs, should have been "platinum". Suggests coil, coil wire or fuel injector. Switches coils to try and isolate problem. Says if the code returns as misfire cylinder two then the problem is probably a fuel injector. If it says misfire cylinder one, then it is the coil or wire. I drive 20 miles and try to get check engine light to go on after his reset, will not go on. SO he suggests I continue on my trip and have next mechanic check the codes and continue the diagnostic. Drive home slowly, car acts OK. Last 50 miles try acellerating, no misfire. INteresting. By the way, I filled up with Premium. So I am home now and will take it to local european auto mechanic. Apparently these symptoms are hard to diagnose. What do you think it is? Plugs, wire, coil, bad gas, fuel injector, fuel filter, catalytic converter, fuel pump, or?