1991 firebird 3.1L V6 (obd1 code 33)

Hi, I need some help with my firebird. A couple of weeks ago when the weather got hot I noticed that my car was sputtering hard and it wouldn't start. I did the paperclip trick in the DLC connector and it flashed code 13 and code 33 on the check engine light, which are the following:

13. Oxygen sensor signal stays lean during warm engine cruise

33. High voltage (low vacuum) at mass air flow sensor (or MAP sensor)

I used the starter fluid trick on the vacuum hoses and found no cracks, we replaced the engine coolant sensor and after I let the car cool off for awhile I started up the car and it seems to idle fine between 600-700 RPMs and I let it run for about fifteen to twenty minutes and then I got back in the car and revved the engine near 4k RPM and it started to sputter again and then died. It flashed a code, this time just code 33. Apparently the new throttle body sensor got rid of the code 13 and we are now trying to find out what's causing the code 33. It only seems to do this when you give it any gas and the car has warmed up to 220 F. Any ideas as to how and where to start tracing this problem?

A guide I found on the firebird forum for code 33 (which I can't understand):

http://members.shaw.ca/corvette86/Code%2033.pdf (http://members.shaw.ca/corvette86/Code%2033.pdf)


Any help you can give me would be greatly appreciated! Thanks