87 300E obstruction to freon flow?

[lizem100]

Was at a tech session this Sat. Looked at pressures on 134a AC(converted a few years ago.) 40 on low side and 250 climbing to 300 on high and pressures dropped whenever fan started-cycled appropriately. Ambient temp 80F. Evacuated system and had .96 lbs-recharged 12 months ago with 1.92. Recharged with 1.90 lbs and same pressures. At idle did not get very cool- 50s. Tech said there must be an obsruction to flow. What would cause this? Receiver drier prob? Of note is that when I went home going 60 mph the temp came down to 40-42F with an ambient of 86F.