Start by checking the signal level meter built into the Tivo.
See Tivo's help page to access the signal meter, readings should be from 80-99 for reliable reception. Check every channel with the up / down buttons on the remote. If it's pegged at 100% on every channel you might have too much signal which is just as bad as a low RF signal level.
Then you should get a tech out to troubleshoot your poor RF signal to the TV outlet.
It can be an outside issue with the drop or fittings, or inside with excess spitting or coax related issues.
And the fact that it works ok with the hd converters? Our other 2tvs with the converters have no problem. Does the tivo need a signal booster
It could be the Tivo's tuner is failing. I doubt a booster will help. Boosters are designed to compensate for the loss in the splitters feeding the TV outlets, and not a band aid to fix another underlying issue.
Without putting a signal level meter on that TV outlet to confirm the proper RF signal strength across the complete band of frequencies, it's all a guess in the exact issue, if it's the signal at the TV outlet or the Tivo itself.
started new thread