This doesn't seem to be a problem related to signal strength issues. When the signal strength is low or more accurately when the Signal to Noise ratio (SNR) is low what happens is you get a high bit error rate (BER). When the D-Link router (or really the 802.11g protocol) detects a high BER it changes the digital modulation method to a different method that is more robust to low SNR. This lower modulation scheme means a lower bitrate which is why when signal is weak you see your speed drop to even 1Mbps. This should never lock up your router unless the signal is lost completely but it should resume when it detects the signal again. Also this has no correlation to size of download file other than perhaps making packet collisions and repetitions more noticeable.
Many things could be causing this including network timeouts. I have also seen this with some routers trying to access certain website. Have the default values for RTS threshold and Fragmentation threshold been changed? You might try to lower your fragmentation threshold to see if that helps. See http://support.dlink.com/emulators/di524/adv_perform.html on where to do this. Here is a nice explanation on RTS/CTS and Fragmentation: http://www.wi-fiplanet.com/tutorials/article.php/1468331
Consider using a download manager application such as Windows based FlashGet or GetRight or iGetter (MAC) which breaks up the download stream into parallel streams. This is often a much faster and simpler solution that trying to rootcause this.
Hope this helps. If anyone out there can clarify this pls would appreciate it. Have to admit were are not really networking guys but rather Radio-Frequency/Wireless guys
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