Man it's been a long time since I worked on one of these, but in reading the thread something came back-read on.--The vane pump is rebuildable, IF the insides of the housing aren't scratched or gouged, and the bushings are still tight-a set of vanes and o-rings and a seal, I seem to remember. You really need to check that pump before you dig too deep into the hydrostats.--What I remembered is trouble we had with the cold-weather bypass poppet in the manifold. There is a plug, spring and poppet in the manifold block, check the poppet for burrs, scratches or a wear ring, anything that keeps it from seating. We were sometimes able to repair the poppet by chucking it up in a drill press and polishing it with emery cloth, but it's good to replace if you can. Anyway, something to check before you pull your hydraulic pump. Tazz is right in all, but a worn hydraulic pump can work OK until the oil reaches operating temperature then have these symptoms. Your problem might be any combination of things. Anyway, check all this stuff, if you're still having trouble, time for hydrostat tests, I guess.