O.P. Gauge dies when towing
 

I have been pulling a trailer quite a bit here lately, and I have noticed something that doesnt happen when empty. When I am pulling a load, and I come to a stop my oil pressure guage goes dead for a few seconds. THen it comes back on. The check gauges light and bell come on too. It only happens when I come to stop while towing.

I am running Delo oil if that makes a difference.

Any ideas?