Recently, my '94 850 Turbo wagon (108,000 miles) has exibited a problem starting, and seems to be more evident when the air temp is hot. When you first crank the engine, it acts like a bad battery - it cranks slowly and then finally catches and fires up. Once running it seems to be fine. I checked the battery (a no maintenance Volvo battery) and found at least one cell was low. I added distilled water and ran the car for a few weeks and checked again. A cell was once again low, so I replaced the battery with a new one from the dealer. I still have the problem. I just replaced the distributor cap, rotor, plugs, air filter and fuel filter. The problem remains. When the engine cranks over, it still cranks slowly, but actually seems to increase cranking speed slightly (not what you would expect if it were a bad battery) just before it starts. It has not failed to start, just cranks slowly. I checked old posts and someone had a problem with the fuel pressure regulator, but I couldn't identify the symptoms as being the same as mine.
Any ideas?

Thanks in advance!