1987 or 1988 suburan timing

i have a 1987 chevy suburban that was built in 1988 that has the throttle body fuel system on it. the timing chain and gears along with the plugs wires cap and rotor have been replaced. it will not stay in time one minute it is fine the next it runs rough miss firing and popping the base time has been set by cutting the tan with black striped wire at the ecm and it will hold when you reconnect it it goes 4 degess out but does fine till you put it under load that is when it goes out then back in. i cant find the manual for this truck it is not a 87 or a 88 but the original sticker says that it was order like this all numbers match up it is the original engine. please help!!!!!!!!