I would start out checking the swr on the antenna, get an analyzer if you can borrow one from someone and check it first, antenna problems aside the built in tuner on the 450 is a compromise, its not good at handling a large mismatch. Some reviews say 3:1 is the most it can handle.
If you don't have a good balun as some have suggested get one!
A plain jane 949 tuner from MFJ is actually a good way to go, i had one for years, i bought one off ebay that had a dead meter according to the seller, turned out to be blown protection diodes. Replaced the diodes and the meter worked perfectly and after fixing a few bad solder joints the tuner worked flawlessly for years. After i added an amp, the SB-200 i rebuilt from the chassis up, i got a Heathkit SA-2040 2kW roller inductor tuner, and i love it, will tune just about anything. But hey, that's not really the point here, i prefer to have a resonant antenna in the first place, but we all know its nice to have a tuner for excursions out to one end of the band or the other.