It sounds to me like you need to backwash, my pressure will do that too as my filter fills up with junk. I assume that when the pump is turned off, the junk re-settles in the filter. When the pump is turned back on, there a new path for the water to take through the filter so the pressure is less. This is fairly short lived though and pressure rises fairly quickly. In reality a backwash followed by a minimum 30 sec rinse is needed. You should be backwashing after a 25% increase in pressure from your clean pressure. If you don't know what that is, just go ahead and backwash, rinse, then switch back to filter and record your "clean" pressure. Once this increases by 25% then backwash. For instance, if your clean pressure is 20 psi, then 20 + 25% = 25 psi.
The chemistry is really the most important part. With a proper test kit and a few questions here you can eliminate the algae and significantly extend your time until a backwash is needed. If you don't have a test kit I highly recommend getting one. Good luck!