I have a 1999 Chevy Suburban K1500 4 wheel drive. It has a 5.7 liter engine and has 360,000 miles. I recently had a fuel pump put in last year. It is now running sluggish when I accelerate it will not go. This happens after it seems to warm up about 15 minutes of driving. I just replaced the fuel filter thinking that it may have been plugged up but it is still running poorly. When I put the fuel pressure gauge on it, it read 61-62 during start up and then dropped down to 57-59 (pulsates rapidly) after I started it. Any advice would be greatly appreciated.