I've got an old truck that has been giving me transimssions problems in the last year (last 10k miles). It happens every couple months. The weird thing is that it's back to normal once I start the truck the next day. When it happens, at every shift, it feels like it drops into neutral before finally shifting a couple seconds later. The truck is basically undrivable when it happens. The last time it happened the O/D off light started flashing and I couldn't shift onto overdrive. I'm going on a long road trip with the truck in a month from now and wanted to ask if any of you guys had similar issues and what was your solution. Thanks in advance!

 

O/D OFF light flashing means there's a code or codes which you must read.
Have you checked the fluid level? This is a solid first thing to do.

Without reading the codes, the only thing you can do, besides external examination, is drop the pan to see what condition the fluid is in.
Have a filter on hand. Buy one with a gasket. Also, you'll need 5 qt. of MerconV ATF.
The fluid could be low or it could be very dirty. Filthy fluid can impede operation as can a clogged filter.

You would be very lucky if a "blind pan drop and fluid refresh" cured your ills, but you wouldn't be the first person to find some joy by doing this
.
Get those codes read. Write them down before you or someone else erase them.
Do not be surprised if the "loaner scanner" offered at an auto parts store can't read transmission codes.

 

Georgeandkira thanks for taking the time to respond. My scanner tells me "p0733 incorrect 3rd gear ratio", which isn't very useful since every gear struggles when it happens. The fluid level is good (tested on a level surface, while hot and running in park), and seems in good condition to my untrained eye and nose (doesn't smell very burnt). I could drop the pan and inspect the filter, it seems like the next logical step.