The MiG-17, The MiG-15, and even the F-86 Sabre (which sports full-boosted controls), all suffer from the same handling quirk at high speeds, in that they'll drop a wing unexpectedly.
This occurs on the MiG-15 and '17 between 450 and 500 knots, and on the F-86 between 550 and 600. It is recoverable on all three birds.
What Jug's referring to is the elevator "freezing" or locking up above 450-500 knots on the MiGs, and that is due to (as Gepard stated) a lack of fully boosted controls on the MiGs. The early 17's had a telescoping stick extender built into the control stick to allow the pilot to exert additional pressure through increased leverage.
The Polish-built Lim-6 (a variant of the '17) is favored by collectors of old East-bloc aircraft, because its fully-boosted flight controls eliminate this.
The Sabres, due to their all-moving tails, do not suffer from this problem.