89 Volvo 740 Turbo won't start when the weather is hot or damp.

My 89 740 turbo has not been starting when the weather is hot (above

80 F.) or when the humidity is high. It will turn over fine, but not start. When it's cooler, it starts and seems to run well. I suspect a fuel problem (pump, filter, etc) but was hoping somebody on this newsgroup may have run into the same problem. How difficult is it to replace the fuel filter and/or pump? And are there any good online resources that provide instructions.

Thanks,

Tom Dirks