As the pumps get older, the oil pressure bypass valve can become gummy with junk. I've seen this on some high mileage engines or ones that had a lack of oil changes. The valve is held in by a circlip and the whole pump can be disassembled then cleaned. If you have the pan off, it's worth it to remove the pump and clean it along with the pickup. Plus you might want to change the o-ring for the pickup.
Also, you can just bump up the weight of the oil in the summer and see if it takes cars of the problem.