i just bought a 1976 vette. it runs great , except when i am going down the highway at about65-70 , just cruising and i decide to kick it up a bit. well when i do this i give it bout 1/4 more throttle and the valves will clatter a little then clear out. anyone have any idea's ? i am running mid grade octane about 89. should i go 93 octane and see if that clears it?. or maybe the timing is a tad too advanced ?