carrier 3 wire pilot gas furnace random lockup, pls help

hope someone here could offer some hints what is next for me to try. Sorry for long description
old Carrier gas 394GAZ048125AAEA, 3 -wire-igniter furnace (bi-metal flame s afety switch) . Several contractors were unable to pin point the problem. T he problem is it randomly locked up. it works for few days no problem. but every few days, it would lock up like this: call heat, sparked, pilot ligh ted up, main burner light up. At this moment, if the spark continues, then the whole thing is shut down and locked up till I flip the power switch o ff/on; If the spark stops, everything goes on no problems for heat cycles.
here is what has replaced so far: new 3-wire safety burner LH680005, new sp ark electrode, igniter/lockout control grey box. 2 limit switches are che cked close all the time. new thermostats, new air filter, new red spark wir e. All the possible parts are replaced suggested by contractors. so I r un out of any ideas as what else to check or replace? seems all the releva nt parts are replaced with new ones.
in short, the problem is if the spark continuous after pilot light up follo wed by the main burner lights up, it will shut down and lock up for good; i f sparks stops after pilot on, all are normal. This lockup occurs very few days. most of time the furnace works fine.
pls shield some lights on the problem, I spent so much time and are unable to get it fixed. thank you in advance for any comments
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