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Crashes in 40 columns but none in 80

I found a rather peculiar issue when it comes with dealing with BBS Telnet software. In 40 column mode I am often seeing RS-232 breaks in teh telnet window long but these do not appear in 80 column mode.
Is there something about 40 column mode that impacts the user port?

Hmmmm are you running 80

Hmmmm are you running 80 column mode with the 8502 at 2 MHz or 1 MHz?  If you're running at 2 MHz, that may explain why it performs better using 80 columns.

I am assuming it is running

I am assuming it is running at 2mHz as it is in 128 mode. I wasn't aware there was a way to switch between 2mHz and 1mHz without executing the "fast" command.

One way to get to 2 MHz mode

One way to get to 2 MHz mode is by poke 53296,255 or return to 1 MHz by poke 53296,0 (using BANK 15)
(Assuming you're using a real 128 (not an emulator like VICE), if you see anything other than just solid color on the 40 column screen you're using 1 MHz mode)
 

Ha. IN 40 column mode I see

Ha. IN 40 column mode I see colored static. Nothing solid.

Yep.  Your C128 is using 2

Yep.  Your C128 is using 2 MHz mode when using 80 columns.  That is most likely why it performs properly when in 80 col mode but not in 40 col mode (because the processor is in 1 MHz mode).  That colored static confirms that it is at 2 MHz without using BASIC's FAST command, but rather POKEing to the aforementioned location to achieve 2 MHz.

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