Need help here. I am at a loss. I have several express 2500 series work vans in our fleet, and this one is throwing me for a loop.

Van fried a fuel pump about 6 months ago, so we replaced it with a new pump (non GM type) also installed new gas cap, with no issues. over the passed few weeks. this is what is happening.

Runs PERFECT if the temp outside is NO HOTTER than 70 degrees. no stalling or randomly cuttng off.

NO STALLING if the vehicle has a 1/2 of tank of gas OR MORE, Stalls ALL THE TIME at radom times if it has less that a half a tank, and it is almost undriveable if it gets below a 1/4 tank. The fuel pump comes on when it starts, and runs the entire time, UNTILL IT STALLS.. vehicle runs, then everything shuts off at the same time (engine and fuel pump)

When you are at idle, with a full tank of gas, when it is hotter than 85 degrees, AC ON. pump will get louder and louder, then it will shut off and engine dies. it always starts right back up after you turn the key completely off, then restart...

Check engine light will light, then shut off after a few starts, We pulled the code and it sais (low fuel rpessure switch) Also, when I pop the gas cap to fill it up, there is no hissing noise at all. Sometimes when I pop the gas cap, it immediatly shuts off the engine.
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Any ideas will be greatly appreciated..