I don't want to be a pessimist but I had/have a similar problem. I can accelerate nicely from a low RPM to cruising speed if I do it very slowly. If I put my foot into it it sputters and coughs and backfire through he carb. When it gets to speed it runs smoothly.
Problem? Failed cam. I have removed the cam and sure enough all but two lobes are rounded and scuffed. It seems to be common these days with the "new API oil standards". They removed all the zinc and phosphate (ZDDP), which is hell on catalytic converters. Problem is the flat tappets, need the extra lubrication.
If this sounds right then stop driving and check for medal in the oil filter. Cut it open or just pour out some oil though the filter intakes. If it looks like veryfine medalflake paint, then thats probably your problem.
Hope for your sake I'm wrong.
Bill
Inliners # 1783
