Is your FM tuner affected by interference?

I have a Magnum Dynalab MD90T tuner that seems to be affected by, what I think is, atmospheric interference. It seems that when the sun is setting my signal strength goes down. I have noise that doesn't allow listening, and it doesn't matter what antenna I hook up. I have had a Fanfare whip, an amplified antenna, and now the Godar, which gives me great DX capability, and nothing cures the problem. It only affects the college stations at 90.7, 91.1, and 91.5. I live on the tallest hill in the county and the stations are 10 miles away. One xmits at 10KW while the other is at 5KW. They both peg the signal strength meter at 9, while the station 60 miles away is at 8.

I recently had the unit back at Magnum' for repair and they found a bad F connector along with a bad multi-path chip. It came back and it sounded better than it ever did. It has performed flawlessly up until yesterday when I noticed the 90.7 station had the signal strength meter bouncing up and down with the multi-path going up when signal strenth was down. Normally, I have no multi-path. I call other friends for radio checks and they get good, clean signal with no noise while I suffer.

Should this unit go back to Magnum Dynalab or do I have other issues?
try a directional beam yagi-type FM antenna

your multipath issues are certainly not a fault in the tuner - I experience similar interference under certain conditions similar to what you've described, even with the yagi
My tuner is stepped-on by seasonal solar flare activity for a couple of hours a day. It begins around 11:20 and is gone by around 1:40. I have not noticed it in the summer.
I was also thinking that co-channel interference could also be bothering your reception; again a directional antenna is worthwhile trying.
I think a yagi would be the answer if my other tuners were experiencing the same problem. My Kloss Model One table radio is not affected, and my Son's Charlie, The Tuner has no problems. The Charlie is operating on the same Godar Model 1 that the Magnum Dynalab has. As far as multi-path goes, we don't have any problem here.

In regard to a nearby station interfering with the channels, there aren't any high powered stations to provide adjacent channel rejection problems.

Is there a possibility of other civil channel local transmissions doing this? I would think the Charlie and the table radio would be affected more than the M.D.. But then with a better sensitivity on the M.D. that might account for it.
I'm not referring to either local 'adjacent-channel' or high-power 'desense' types of issues: co-channel interference means receiving another station which is broadcasting (from a further distance away) but on the *same* frequency as your nearby station of interest, thus interfering. A higher-sensitivity tuner will be more subject to that type of degradation than a lower-sensitivity device. I was thinking that a directional yagi antenna should be able to help reject such interference, unless of course both stations are basically along the same geographic direction.