#87
To confirm: You're checking your console at the main menu once you've launched the game?
I ask to rule out any class-specific cfg commands you may have in the \my_custom_stuff\cfg\ folder that could change the value upon class selection.
Easiest way to determine what's wrong will be to post the contents of your autoexec.cfg.
Sidenote: I think it's "better" to set your interp to "0", and use your cl_updaterate/interp_ratio values to lower the interp automatically. A cl_interp of "0", an cl_interp_ratio of "1" and an cl_updaterate of "66" would result in an in-game interp of 0.0152
Back on point:
It's also possible that there's something in your config.cfg overwriting the values from your autoexec.cfg. In addition to this, it could be your Steam Cloud config.cfg, or the local-Cloud config.cfg.
We can consider this if nothing is obviously wrong with your autoexec.cfg
To explain:
In the past I had a bunch of problems with the Cloud-stored "config.cfg" overwriting newer settings (config.cfg would soak up settings from demos I watched), so I decided to disable Cloud storage for TF2. In the process of investigating this I discovered that Valve also have "local" versions* of the Cloud storage files. Found here: Steam\userdata\<your_account_number>\440\remote\cfg\
*Only in the absence of this file will Steam download the actual Cloud file.
Note: 440 is the appid for TF2.
If you want a totally fresh start, disable Steam Cloud for TF2, delete "config.cfg" from Steam\steamapps\common\Team Fortress 2\tf\cfg AND from the local Cloud storage folder I mentioned above.
Steam will automatically generate a new config.cfg upon launching TF2, populating it with any non-default values contained in your autoexec.cfg
Important I: Any custom settings: for example fast weapon switching, graphics settings, download settings, and so on, will be restored to their default values if they're not explicitly stated in your autoexec.cfg.
Important II: TF2 has introduced a bunch of fairly annoying pop-ups for "new users" relating to tutorials, backpack instructions, and who knows what else.
Because you're starting with a new config.cfg, the game will pop these up again. Fortunately, each of these has a console command associated with them, which can be added to your autoexec.cfg, making the process entirely painless.
Let me know if you'd like me to post them here, instead of having look them up for them yourself.
I could actually post my own autoexec.cfg for reference purposes. It doesn't have much in the way of graphical commands, but most everything else is covered.