and it gets "WEAK" between 30 and 55 mph

 

If the cat is clogged, you don't need to wait for the car to stall to see if it's getting cherry red, and it's best to keep the car running while you take a look (just so the color due to heat does not fade away). 15 minutes is just to get the car to operating temp and you can then inspect the cat.

Even though the previous owner had it changed, this would still be worth while to inspect.

Also, have you cleaned that EGR tunnel yet?

 

Several members have had problems when they go up hills and the like, they say the power disappears, they replace the cat and it fixes it. Again, if the exhaust gasses can't get through the piping and the engine can't get rid of the exhaust (or it backs up into the engine) it's going to kill your power.