Chevrolet Repair: engine trouble, chevrolet z71, gas additive

Question
I have recently bought a '98 Chevrolet Z71.  The only modification that it has is a "Jacobs Electronic System" installed on the engine.  While driving, the engine seems to "diesel"  for a few seconds when I reach a speed of  approximately 35 mph.  Also, sometimes the engine seems to start "hard."  I used a gas additive to clean the fuel injectors, but that didn't seem to help any.  Any advice that would steer me in the right direction will be greatly appreciated.

Sincerely,
J. S.

Answer
Hi J.S.
I don't know what you mean diesel at 35...diesel is used to indicate the engine continuing to run after the ignition is shut off, without the spark, ie, like a diesel.
If you mean rattle, with rattling, and hard to start, I might think the timing is advanced too far.
Van