I have a 1995 chevy silverado 350ci engine, a.t. w/ 158K miles. 6 months ago I had the intake gasket replaced and then again 3 months later at another garage. the truch has been idling rough and the mechanic raised the idle to compensate to no avail. @ approx 54 miles an hour & 2k rpm the engine surges briefly to approx. 4oo rpm. when the 2nd mechanic changed the intake gasket he spent 2 weeks & 500 miles trying to get the gunk out of #7 which has 75#'s of presure, all the other cylinders have 120#. any suggestions?