Modular phone line problem

I have two lines in my house and up until about a week ago everything worked fine. We have the computer using one line and the phones all use the other. We have 5 jacks in the house. The computer and a phone share one jack with a splitter.
Anyway, about a week ago when the phone on one particulair jack is used, about 12 seconds later, the computer looses it's connection. This only happens with one jack. In troubleshooting this thing the phone sharing the splitter with the computer does not dissconnect the computer nor does any other phone jack with the exception of this particuliar jack.
Also in trouble shooting, the computer has no effect as I set it up with a regular phone in lieu of the computer and the phone went dead too.
Any suggestions would be appreciated. Thanks.
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