wierd am/fm antenna reception prob

We have a 98 Trans-Am in perfect shape with only 29,000 miles. Entire car is virgin. The speaker surrounds, however, have dry rotted and we are removing the entire Monsoon system and replacing everything with new speakers. The head unit is a Kenwood 138. With the new h/u, the radio reception picks up clear at low volume levels, but when you crank up the volume, the higher you go, the worse the reception gets. We are not using the powered outputs on the h/u, we are using a 4 channel external amp. I tried another h/u thinking the reception part may be defective, and same prob. I tried a higher model, and same result. I have tried 3 different antenna adapters to remove a defective adapter. The factory radio picked up fine. Any thoughts on this? We were going to add a powered antenna booster, but they forgot to order one, and the car is here to be worked on. The display on the kenwood shows stereo (st) on the display when playing a low level and about halfway up, the (st) starts flickering and a little more volume it looses (st) all together! Wierd to me! Thanks in advance!

Okay, I found a powered external antenna. One like you would put on a boat. I am getting the exact same problem. Thinking about a different brand h/u maybe? It seems that the low 90's stations and the 107 extreme upper end stations, pick up excellent. No probs at all. Only the middle spectrum is causing this problem?

After a couple hours of phone calls, trial and errors, etc, we figured it out. As wierd as it may seem, the 4 channel amp was causing the problem?? Go figure. Cd player played strong all the way up. The lower end spectrum of FM played fine and the upper end FM played fine. But when in the mid 90's radio stations faded out with volume. We changed the amp out and that fixed the problem! Now... does anyone here have any idea how or why this could happen? I have been in the audio business for 20 years now and it boggles me!

Frequency of the power supply must put out a harmoic (rf) that the radio was picking up.

As stated earlier, power supply section of some amplifiers do generate RFI, Radio Frequency Interference.  If the amp is mounted near the antenna of the car, if you move it as far away from the antenna as possible, it should take care of the problem.