I just put my 1984 245ti back on the road after a period of storage. Cold it would start up fine but then be balky on hot restart - run with a lumpy idle but drive ok. I couldn't wait to do any diagnosis - a trip to Winston-Salem to see our 13 year old grandaughter in a school opera meant getting on the road. It behaved that way all the way down (that is-going from north to south) and continued when I went to Charlotte to visit the Pullapart there (4 240's in stock). I suspected clogged injectors so my son - a Mini-Cooper tech - tried to do an injector cleaning before I went to Charlotte but someone had messed with the kit's pressure regulator and it dumped the entire mixture in seconds--should have run the car for 20 minutes or so. It all blew out the exhaust when I restarted on the gas in the car's tank--no change in the condition.
On the return to W-S there a good long grade and mostly out of frustration I just let it rip -- got right up to 5000rpm+ (about a 100mph) and held it there till the crest. Shorty thereafter I made an exit from the highway. At the bottom of the ramp it idled perfectly--just like it used to do--and has been perfect ever since.
My Danish Volvo tech friend told me after service they used to run cars (544,122&140 models) in 3rd gear, foot to the floor for 8 miles on a highway--turn around and do the same on the return except for shifting to 4th the last mile or so-with the car gaining another 8 or 9 mph over its "over the top" top in 3rd. Call me a "believer". -- Dave