Oil Gauge fluctuates

I have a problem with my runner. I just had the valve cover, head gaskets replaced, and now my oil pressure gauge doesn;t stay in the same place. When I first start, it will be amost full, then after 5-10 min of driving, it will drop to low, but I check the oil dipstick and full. Also, the car tends to overheat, the thermo has also been replaced last year.

Thanks

I have a problem with my runner. I just had the valve cover, head gaskets replaced, and now my oil pressure gauge doesn;t stay in the same place. When I first start, it will be amost full, then after 5-10 min of driving, it will drop to low, but I check the oil dipstick and full. Also, the car tends to overheat, the thermo has also been replaced last year.

Thanks
Test

test

One source of overheating is placing the head gaskets on backwards. Some of the water passages are then blocked.