I have noticed a hesitation in the engine around 1900 - 2000 rpm. Engine bogs down when driving under load. This happened rather suddenly. I changed spark plugs (very worn), Valve cover gasket (small leaks) & air filter. Can't adjust timing because of computer control. The engine idles smooth & aside from the hesitation revs smooth. I don't hear anything bad mechanically. Not burning any oil either. Does anyone have any idea what the problem could be???