well these EPR systems weren't the best invention IMPCO came up with imo.
But that aside i would check the mixer , clean it out, check the diaphragm as well.
check all the vacum hoses, a vacum leak will cause this.
If you have the PSI Display diagnostic program from impco and the correct can box you can connect with a laptop and see whats going on and get a better idea of where to look. They did make several generations of this can box so you will need to know the Tier Level to determine which one to use.
I know when i worked for caterpillar the Tier 3 early models used one version of the canbox and then they changed it and came out with another version using a different canbox, they were NOT cross compatible either. :o/
I had 2 different versions of the software on my laptop for that one unfortunatly :o/

had a home depot account where they had these issues and all they wound up doing was changing their LPG supplier because they were using a low grade LP Gas that was gumming up the system and creating all sorts of issues.

and then again.....
sometimes just an old fashioned tune up cures this ;o)

yes even the distributor cap lol :oD