I just picked up a 91 about a week ago and I been driving it. She runs good but other than the fact that I think the Tstat is stuck open and not warming up, it sounds like its loping at idle and the cluster lights dim and shine with the lope. What could cause this??

 

Try and bring up the idle a bit, it is probably below the cut-in speed.
The thermostat is easy to replace.

https://www.dieseltruckresource.com/...6&postcount=10.

Jim