im having a problem with my 85 318i can u help. please!

almost exactly at 4300 rpms the car starts cutting out and the tach needle goes nuts. ive put an intake system on it, plugs, dis rotor, fuel filter to try to fix the problem. my next guess is the new rail mounted fuel pump? does anyone have the answer?
thanks and again if anyone has had this problem and fixed it! please!!! please!!! let me know-

1985 bmw 318i 1.8l 4cyl
135,000 miles, old lady owned-

thanks greg
gregorytobin1022@yahoo.com

p.s. the car doesnt do it until the car reaches normal temp, and i could be wrong but if the outside weather is really cold it doesnt do it, but again if you start it after sitting over night and drive it its fine. until it reaches normal temp. what the heck is going on?? please help i dont want to keep spending money on maybe's.... thanks guys!
also if it helps when i changed the fuel filter it was the stock 85 filter etc... so again maybe the fuel pump is the problem, but i dont know if that was the case, y it wouldnt do it at cold or hot temp/