Unstable product

Product bought two copies: one as an AP (1), the other 'client' (150m distance).
Instability in reception: Change in strong signal, constantly interrupting connections
After replacing the AP by a competitor product AP (2) N standard, slight improvement but still problems disconnect / reconnect with appalling speed.
After also replaced the client DAP 'by a previous product (G standards only), return to 100% stability (force) and speed multiplied by 10 (with the AP (2)).
Straw!