Need help with a 1997 Gmc!!!!!!

[ldhwade]

We have a '97 Gmc Sierra...305 vortec 8 cylinder.

We were taking it to get the transmission rebuilt the other day. It was being driven in 2nd gear at approximately 50 mph. All of a sudden the engine just acted like it lost power. You pressed the gas and nothing. Pulled over killed it and let it cool down a little. Went to restart it and it won't start. When it lost power the SES light came on and threw codes
p0108 which is manifold absolute pressure/barometric pressure circuit high input and P0146...O2 sensor circuit no activity detected (Bank 1 Sensor3)...the second code had came up before but it's been awhile. So does anyone have any suggestions as to where to start looking to try and figure out what is wrong.

Thanks in Advance.

[ldhwade]

I forgot to mention...that it is getting gas to the motor.