Started to miss/stumble

1994 Toyota 4 Runner mileage: 174,000. My 4-Runner has started to miss/stumble when accelerating, doesn't seem to miss/stumble once you reach a constant speed, doesn't miss/skip when idling, just had it tuned up not to long ago (it should not need another one this soon). When going up hills there is a strong smell like sulfur/rotten eggs, I think it's my catalytic converter, short of replacing it is there anyway to unplug one? It also leaks oil from front cover, could the clogged converter cause this oil leak? Also, I have had my exhaust welded several times seems to keep getting holes in it always before the catalytic converter, could that be due to a plugged catalytic converter? What would be a reasonable cost to replace converter? Ballpark figure, can I buy a used catalytic converter from the junkyard? Is there a way for my mechanic to check for plugged catalytic converter without drilling a hole in exhaust? Probably wouldn't have to drill a hole since it already has one! Could it just be a problem with fuel mixture, it is fuel injected.