Not sure what's going on.

I have a 1998 Ford Explorer that worked fine, we went on vacation no problem. When we got back the truck stayed parked for 2 days and then started doing the following:
fluttering when at a red light or stop sign
then it started acting like the filter was stopped up
2 days after that I only had 2nd and 3rd gears.
We were told from the codes that came up that it could be my computer.
Is there any way that we can have the computer tested before buying a new one?