Filter Pressure Variation

[Pool Party]

I have a Hayward S-200 sand filter with a 1 hp motor. The shell is very old so I have a valve between pump and filter so I can regulate pressure down to 10 psi after a backwash. This year I have noticed a jump in pressure as the day goes on, sometimes up to as much as 10 psi from the reading when I turned the filter on in the morning. If I turn the pump off for awhile and than turn it back on the pressure reads normal again. I've replaced the gauge but this didn't fix the problem. I know the pressure gauge is working properly because the filter will start to leak water as the pressure gets higher. (due to small stress cracks). When the pressure is high ( 20 psi), the discharge into the pool is strong. (feels like it does at 10 psi) Typically if the pressure is high the discharge should be reduced as it has been in other years meaning I need to backwash. I've replaced the sand last summer. I've run out of answers and I need to find a solution as I don't want to destroy my filter shell. Hopefully someone has experienced this same problem and can help me. Thanks in advance for your help