1976 510 While Using The Backhoe Sometimes It Just Has No Power To Dig. I Have Found That If Trying To Curl The Bucket Or Uncurl If I Just Hit The Boom Lever Up Or Down It Will Get More Pressure To The Bucket For A Second. Same Thing With The Dipper. If You Try To Use Just One Sometimes It Has The Power It Should Other Times It Will Just Sit, Does Not Lug The Engine At All. Doesnt Matter Engine Speed. I Put A Pressure Gauge On Pump It Has Just A Little Over 2000 Psi. Have Changed All 3 Hyd Filters. Replaced All The O Rings On The Spool Valves And Looked At Valves No Scarring Or Wear. Steering Is Easy. Can Pull Into Dirt Pile With Diff Locked And It Will Spin Both Rear Tires Without Killing Engine. Any Ideas???