[lineslinger]

Electrical, you replaced the fuel pump and now it still demonstrates the same tendancies a thousand (plus or minus) miles later. You said it yourself, like someone turning a switch on and off. Thats the same as a bad electrical contact, sometimes you have current and sometimes you don't.
Nothing like a intermitent electrical problem. Distributor drive gear?(slow and progressive due to wearing down) Loose ground? Belt too tight on the alternator(burned bearings). To me it sounds like your distributor drive gear is wearing out/down, what kind of cam are you running? A lot of places to look, take your time, one step at a time.

Mark