Fuel pump replacement, now engine and RPM acting weird. HELP

I recently had my original fuel pump replaced at around 340-345000 miles on my 1995 toyota tacoma xtd cab, 3.4L 4X4. The original was in there before and for a week it wouldn't start up. now that the fuel pump is replaced it starts up fine but acts weird when its running.
1.) When I start it up and it gets warmed up, it Idles right at 2,000 RPM, before it idled around 1,000 or 1250
2.) No problem when im actually moving forward, but when I'm in neutral and applying the brakes and above 20MPH, it revs up and down between 1,250 RPM and 2,000 RPM and goes back and fourth up and down until exactly when I hit 20 MPH, then it jumps back up to 2,000 RPM and stays there (just like when im at an idle stop). If I let off the brakes though while I'm above 20MPH and still in neutral, it stops hovering between 1250 and 2000, and stays right at 2,000 just like when its idling, but as soon as i touch the brakes and am over 20MPH it will jump up and down.
3.) I dont know what it is and really would like to get it fixed because I plan on keeping my truck of course, its just the gas mileage has dropped to somewhere around 14-16 MPG (normally 21 or 22).

PLEASE HELP. anyone who has had to change their fuel pump or has any idea what this problem is (electrical, fuel pump regulator etc.) or what I can do please let me know. I would GREATLY appreciate it.

All the best

 

clean your MAF sensor?

check your fuel lines for any leaks (probably not it but worth a look over)

did you double check all your connections before putting the tank back up?