1987 Cougar. Need help.

[GT-R SkylineX]

My 1987 cougar is acting werid and i if anyone knows or has a idea whats wrong i would really like the help.

When driving it in 1-2 gear it worked fine. but once it got to 3-4-5 gear it starting slowing down radpidly then jumping forward. Going up hills it would die and have to roll down and get restarted. Stoping at lights it usually died.
We replaced the spark plugs and fuel filter but nothing changed. flooring it only got to about 40mph.

We think it is the fuel pump not sending enough to the engine. Can someone tell me if we have the right idea or really far off?

Thanks