I agree with Ronnie. With the IPC, radio, and HVAC units all failing at the same time it does sound like a loose ground connection. Also, he might pull the IPC from the dash and clean that wire connection.
If I'm not mistaken, I don't believe any mod to lower the Reatta seats exist without major surgery (cutting/welding/etc.). At my age I've shrunk down to below 5'6", so no headroom problems for me (although I'm 6'2" in my own mind)!
When it won't run, check fuel pressure with a gauge and check a spark plug wire to see if you have spark. Let us know what you find. If you don't have a fuel pressure tester gauge you can borrow one from an auto parts store.
I'm not sure about the '91 models, but on other models the electronics have dedicated ground terminals near the battery that have been known to corrode and/or get loose. Since your problem is intermittent I would start by cleaning all grounds to eliminate that as the cause of your electrical issues.
More information about it can be found here:
Reatta Owners Journal - Ground Terminals & Junction Box Location