I have the same problem you do, multipath hell in both my office at work and my home office in the basement of my house.
Magnum Dynalab offers two antennas that may meet your needs. First up is the inexpensive and easy to place SR100 Silver Ribbon Antenna ($30):
Silver Ribbon Tuneable Antenna
-Continuously tunable across FM dial
-Helps reduce multi-path interference
-Directional for best station orientation
-Receives HDTV broadcasts with its UHF design
If that doesn't work, take it back to your dealer and exchange it for a ST-2 Omnidirectional Whip antenna ($100).
Omni-Directional AM/FM Antenna
-Vertical 1/2 wave design
-Rugged stainless steel construction
It's a whip antenna you mount on your window frame and comes with a 25' wire to run to your tuner.
Finally, if you still have problems you can consider adding a MD Sleuth FM Signal Amplifier ($350):
FM Signal amplifier
-Antenuates strong, harsh FM signals
-Amplifies weak FM signals (up to 30db)
-Separates interfering adjacent-station signals
-Aids in reduction of multi-path problems
Check out http://www.magnumdynalab.com/home.html
No, I have nothing to do with MD, just know this stuff works and is reasonably priced. Good luck, Jeff