Poor acceleration after slowing down

I have a 99 Grand Prix GT with a 3.8 L V6 and 188,000 miles (remanufactured engine has about 50,000 miles). About a year ago I noticed a new intermittent hesitation problem at low speeds while the engine & transmission are hot. The problem has increased in frequency. It typically happens right after I stop or slow down and then I try to accelerate. The car will start to go and then the acceleration flattens for a few seconds and then something kicks in and it accelerates like it's supposed too. This is now starting to occur intermittently at high speeds. It's getting dangerous since it usually happens when I'm turning in front of traffic.

My transmission was going bad, so I had it rebuilt a few months ago. It was shifting hard in all gears, now it is only shifting hard from 3rd to OD. Not sure if it's related to the hesitation or not.

My car has had all proper maintenance. Here are some things that have been replaced since the problem started: computer, Intake Gasket, blower motor, spark plugs, spark plug wires & ignition coils (again). No warning lights are on. My mechanic has checked the codes twice already but the issues did not fix the problem. I'm going to get it checked again for new codes next week and I'm trying to do as much online research as I can.

Any suggestions would be greatly appreciated :)

Is the temperature normal? What is the temp gauge at the IP reading. Have you checked fuel pressure?

Fuel pressure or I would also suggest an Ohm sweep on the TPS sensor just to rule it out.

Thanks for the responses. The temperature runs at 185 and jumps to 210 quickly when I slow down, higher if I sit too long. The temperature has increased over the passed couple of years. I used to run at 160. I had the fuel pressure checked awhile back, but I will revisit that one.