***UPDATE*** So I managed to track the original owner down of the car and he said the issue has to do with the harness running to the ECU. He said if I shake it, should start right back up (it didn't but besides the fact). So how would one go about fixing this issue, I do not even know where to start, there is 3 harness connected to my ECU all with like 20 wires attached to them running god knows where. 1990 EF shell, b20b block b18 head (unknown ecu will check)

Hello everyone, It has been a long time since i have been in the honda game and now had a deal too good to pass up. The issue I am having though is with regards to starting. So I can start it a few times in the day and all is well no issues whatsoever and then out of nowhere I come back from getting a soda at the gas station etc and bam nothing. All the lights come on, fuel pump clicks etc but just dead doesn't turn over nothing. I had my alternator, starter and battery tested at auto zone the other day and they said all the test came back 100% good to go. I had originally thought it maybe was my clutch safety whatever its called ( i usually bypass those anyways because they are annoying when they go out ) but when i hook up jumper cables it will start immediately. My terminals are all good, I checked all the wires from my starter, alternator, battery etc and am going crazy trying to figure out what the hell this is. If there is anything any of you smart guys and gals can think of please let me know or if i need to recheck something please offer up any ideas that come to mind. I am no mechanic at all, I know what parts to buy that make what #s etc I want out of my car but I am never the one to put on lol. Thanks in advance and I am super excited to be back in the honda game and back on the forums (even though its for tech help but still glad to be back).