I just spent the weekend dealing with the exact same symptoms. My '87 240 has been stalling at very low speeds, at turns, and at stops. Otherwise, there was no problem. I checked the fuel pump relay, it was fine. I pulled and cleaned the entire flame trap to include the lower and upper parts (lower one goes into the oil separator in the block, upper one contains the actual flame trap and has two extending ports) and both connecting vacuum hoses (large hose to the air-intake flex hose and the very small one to the outlet above the flame trap. I also removed and thoroughly cleaned the throttle body with air-intake cleaner (soaked rag - DO NOT SCRAPE) that is similar to carburator cleaner. The actual cleaning itself took about 1.5 hours as it had never been cleaned in 18 years. All of the work appears to have paid-off, the engine has not stalled (fingers crossed).

Good Luck
BudaVol '87 - 215,000 miles